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From what I know developed countries seem to have stayed developed (or at least recovered quickly) even if there was a devastating war, e.g. Germany after WW2. If it's indeed the case that developed countries tend to stay developed, I wonder why - in other endeavours such as sports, if one does not practice, one's skill decreases quickly. That would seem to indicate that getting to a developed state is hard, but getting "undeveloped" is easy. Besides, if it's easy for a formerly-developed state to become developed again quickly, that should also indicate that currently-developing countries should be able to develop quickly, and that's clearly not the case.

The easiest explanation is that my knowledge is wrong. Is it? Were there any developed countries that became "undeveloped" for reasons other than war?

I'd pick Argentina in the run-up to the Great Depression:

During the first three decades of the 20th century, Argentina outgrew Canada and Australia in population, total income, and per capita income. By 1913, Argentina was the world's 10th wealthiest state per capita.

Beginning in the 1930s, however, the Argentine economy deteriorated notably. The single most important factor in this decline has been political instability since 1930, when a military junta took power, ending seven decades of civilian constitutional government. In macroeconomic terms, Argentina was one of the most stable and conservative countries until the Great Depression, after which it turned into one of the most unstable.

Despite this, up until 1962 the Argentine per capita GDP was higher than of Austria, Italy, Japan and of its former colonial master, Spain. Successive governments from the 1930s to the 1970s pursued a strategy of import substitution to achieve industrial self-sufficiency, but the government's encouragement of industrial growth diverted investment from agricultural production, which fell dramatically.

The era of import substitution ended in 1976, but at the same time growing government spending, large wage increases and inefficient production created a chronic inflation that rose through the 1980s. The measures enacted during the last dictatorship also contributed to the huge foreign debt by the late 1980s, which became equivalent to three-fourths of the GNP.

In the early 1990s the government reined in inflation by making the peso equal in value to the U.S. dollar, and privatised numerous state-run companies, using part of the proceeds to reduce the national debt. However, a sustained recession at the turn of the 21st century culminated in a default, and the government again devalued the peso. By 2005 the economy had recovered, but a judicial ruling originating from the previous crisis led to a new default in 2014.

There is a very questionable assumption here that it was "easy" for West Germany or any other European country to quickly regain their positions among the world's leading industrial powers. It is by no means clear that this would have been accomplished without the United States' strategic decision and practical capacity to invest heavily in the Marshall Plan and related policies. I would be very careful to draw any general conclusions based on this rather exceptional case. In fact the stunning economic advancement of certain newly industrialized countries in East Asia during the late 20th century (most notably South Korea and Taiwan) could support the interpretation that similar geopolitical interactions with the United States were more generally a central determinant of economic development status in the twentieth century.

As has been discussed in the comments, the definition of "developed country" status is also a difficult issue. The wording of the question implies a share binary between "developed" and "non-developed" status. Any such definition would be arbitrary, as there is in fact a gradient between middle-income and high-income status.

A different and more systematic approach would be to look at estimates of per capita GDP in general, in order to compare where countries stand in relation to one another over time. An OECD report entitled The World Economy: A Millennial Perspective by Angus Maddison compiles a lot of relevant data for this. One interesting pattern we can pick out of this data (Table B-21, p. 264) is the collapse of the former USSR. In 1973, that region was solidly middle-income at $6,058 per capita. This was roughly 50% of per capita income in the 12 leading European economies at the time, and 36% of that in the United States but over 7 times that of Africa. Looking ahead to 1998, the former USSR was down to just $3,893 per capita, 64% of what it was a quarter-century earlier. In relative terms, the drop was at least as dramatic. The per capital income of the former USSR in 1998 was less than 21% of that in the leading countries of Western Europe, 14% of the United States and less then 3 times that of Africa.

This case stands out in this particular data set as is it is the only absolute decline in per capita GDP I am seeing at all. There is also a significant relative decline in Latin America compared to more advanced economies from 1950 to 1973. In 1950, per capita income in Latin America ($2,554) was almost 51% of that in the leading 12 Western European economies ($5,013). Although Latin American per capita income nearly doubled to $4,531 by 1973, this was a decline in relative terms, down to 37% of the leading countries in Western Europe which had nearly tripled theirs to $12,159 in the same period.

The terms "developed", "underdeveloped" and "developing" are essentially political labels, and very much a question of perspective, i.e. Eurocentric. Much of the policy or debates surrounding these terms lack proper context of local societies and views "progress" (and development) in terms of material wealth (e.g GDP).

This is probably not the answer, nor the place, to go into the nuanced argument against Eurocentric labels in development studies. And so, I recommend a short article from 1995, "Universalism, Eurocentrism, and Ideological Bias in Development Studies: From Modernisation to Neoliberalism" Third World Quarterly, Vol. 16, No. 1 (Mar., 1995), pp. 121-140.

Pushing the logic of realpolitik to its maximum, by definition, "developed" nations are OECD countries that cannot fail. So, the question by OP is a paradox, until we are careful and cognisant of the perspective/bias we are adopting.

I don't mean to say OP is Eurocentric. All I'm saying is historians are generally careful of these labels because these labels are more useful for political purposes, than history. James Blaut's work is instructive here altho' he did not get to finish his trilogy because he passed away suddenly in 2000.

The Ottoman Empire was highly developed in the 16th century and pretty backward by 1850. This was not primarily the result of wars, but of the world passing it by.

I can notice a few examples in all historical timeframe:

The Empire of Minos in the island of Crete was a developed empire, according to antiquity standards, as it includes a true administration, artisanal and nearly-industrial fleet building, and massive trade.
It was possibily leveled by a natural catastrophe, but wars with the Mycenians based in Greece also played a role.

The empires in India and Angkor had successevely decreased and the regions occupied by theses empires sometimes lost infrastructure (ie development). Wars played a role, but internal desorganization and difficulties to provide enough subsidies to the population was also a cause of internal desorganisation.

The African empires that occupied parts of Sahara are not well-known, but it seems that the desertification destroyed their infrastructures and administrations.

The indonesian societies, and especially Bali, were also crushed by a volcanic eruption.

In modern era, we notice temporary lack of developed characteristics, like education, electrical power, fuel supplies and organized military. The ex-USSR in the 90ies and maybe Venezuela today presents these characteristics, but it was only a temporary situation: Today's Russia is catching up with the benefit of the infrastructures, that did not become totally insane after a few years of lack of organisation.

I hope this patch of examples helps. It is mainly about natural catastrophes, but also about internal political desorganisation, where corruption and crime seem more destructive than civil or external wars.

Edit: I notice someone answered at the same time, with more data on the ex-USSR example