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I heard several times from numerous sources, that there were times in history (during the 1st and the 2nd Temples) when the Torah (incl. Oral) was almost completely lost and just one Rabbi (or a handful of Rabbis) saved it all.

Who are they and what sources speak of such examples?

Yevamos (62b):

שנים עשר אלף זוגים תלמידים היו לו לרבי עקיבא מגבת עד אנטיפרס וכולן מתו בפרק אחד מפני שלא נהגו כבוד זה לזה

והיה העולם שמם עד שבא ר"ע אצל רבותינו שבדרום ושנאה להם ר"מ ור' יהודה ור' יוסי ורבי שמעון ורבי אלעזר בן שמוע והם הם העמידו תורה אותה שעה

Rabbi Akiva had twelve thousand pairs of students in an area of land
that stretched from Gevat to Antipatris in Judea, and they all died in
one period of time, because they did not treat each other with
respect.

And the world was desolate [of Torah] until Rabbi Akiva came to our
Rabbis in the South and taught his Torah to them. This second group of
disciples consisted of Rabbi Meir, Rabbi Yehuda, Rabbi Yosei, Rabbi
Shimon, and Rabbi Elazar ben Shamua. And these are the very ones who
upheld the study of Torah at that time.

Rashi:

שמם - שנשתכחה תורה

(Desolate - Because the Torah was forgotten)

In Melachim II, chapter 22 during the time of King Josiah the High Priest Hilkiah found the "scroll of the law" in the temple. This chapter does not imply that it was necessarily the only such copy of the law (and it's not clear whether it refers to the whole or only part of the Torah), but from the reaction of the king we can infer that its contents were not well known, and that it had not been obeyed by the king or people of Judah for quite some time.

The Gemara in Sanhedrin (14a) tells of when Semicha was in danger of being lost forever. But Rav Yehuda Ben Bava saved it:

בטלו דיני קנסות מישראל שפעם אחת גזרה מלכות הרשעה גזירה על ישראל שכל
הסומך יהרג וכל הנסמך יהרג ועיר שסומכין בה תיחרב ותחומין שסומכין בהן
יעקרו

The laws of fines would have ceased to be implemented from among
the Jewish people, as they would not have been able to adjudicate
cases involving these laws due to a lack of ordained judges. This is
because at one time the wicked kingdom of Rome issued decrees of
religious persecution against the Jewish people with the aim of
abolishing the chain of ordination and the authority of the Sages.
They said that anyone who ordains judges will be killed, and anyone
who is ordained will be killed, and the city in which they ordain the
judges will be destroyed, and the signs identifying the boundaries of
the city in which they ordain judges will be uprooted. These measures
were intended to discourage the Sages from performing or receiving
ordination due to fear for the welfare of the local population.

מה עשה יהודה בן בבא הלך וישב לו בין שני הרים גדולים ובין שתי עיירות
גדולות ובין שני תחומי שבת בין אושא לשפרעם וסמך שם חמשה זקנים ואלו הן
ר"מ ור' יהודה ור' שמעון ור' יוסי ור' אלעזר בן שמוע רב אויא מוסיף אף ר'
נחמיה

What did Rabbi Yehuda ben Bava do? He went and sat between two
large mountains, between two large cities, and between two Shabbat
boundaries: Between Usha and Shefaram, i.e., in a desolate place that
was not associated with any particular city so that he not endanger
anyone not directly involved, and there he ordained five elders. And
they were: Rabbi Meir, and Rabbi Yehuda, and Rabbi Shimon, and Rabbi
Yosei, and Rabbi Elazar ben Shammua. Rav Avya adds that Rabbi Neḥemya
was also among those ordained. This incident indicates that ordination
can be performed by a single Sage.

כיון שהכירו אויביהם בהן אמר להן בניי רוצו אמרו לו רבי מה תהא עליך אמר
להן הריני מוטל לפניהם כאבן שאין לה הופכים אמרו לא זזו משם עד שנעצו בו
שלש מאות לונביאות של ברזל ועשאוהו ככברה

When their enemies discovered
them, Rabbi Yehuda ben Bava said to the newly ordained Sages: My sons,
run for your lives. They said to him: My teacher, what will be with
you? Rabbi Yehuda ben Bava was elderly and unable to run. He said to
them: In any case, I am cast before them like a stone that cannot be
overturned; even if you attempt to assist me I will not be able to
escape due to my frailty, but if you do not escape without me you will
also be killed. People say about this incident: The Roman soldiers did
not move from there until they had inserted three hundred iron spears
[lunkhiyot] into him, making him appear like a sieve pierced with many
holes.

text and translation from Sefaria

One case is in Temura 16a (translations from Sefaria), where Joshua forgot 3000 laws (other numbers are mentioned too), and Othniel the son of Kenaz restored them:

אמר רב יהודה אמר שמואל שלשת אלפים הלכות נשתכחו בימי אבלו של משה

...

אמר רבי אבהו אעפ"כ החזירן עתניאל בן קנז מתוך פלפולו

Three thousand halakhot were forgotten during the days of mourning for Moses.

Rabbi Abbahu says: Even so, Othniel, son of Kenaz, restored them through his sharp mind.

Another case is mentioned in Kiddushin 66a, after King Yannay killed all of the sages:

מיד ותוצץ הרעה על ידי אלעזר בן פועירה ויהרגו כל חכמי ישראל והיה העולם משתומם עד שבא שמעון בן שטח והחזיר את התורה ליושנה

Immediately, the evil arose and caught fire through Elazar ben Po’ira, and all the Sages of the Jewish people were killed. And the world was desolate of Torah until Shimon ben Shataḥ came and restored the Torah to its former glory.