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