The Kuzari speaks about this in Maamar 3:54:
Al Khuzari says:
I only know that the people of the second Temple forgot the Tōrāh, and were ignorant of the law of Succāh till they found it written. A similar thing happened with the law that 'an Ammonite shall not enter the congregation of God' (Deut. xxiii. 3). With regard to these two points it is said: 'They found written.' (Neh. viii., 4; xiii. 1). This proves that they had lost the knowledge of the law.
Upon which the Rabbi answers this by saying:
If this be so we are to-day more learned and erudite than they, since we think we know the Tōrāh. [...] Should we be commanded to bring a sacrifice, would we know how and where to slaughter it, catch its blood, skin and dismember it, and into how many pieces, how to offer it up, how to sprinkle the blood, what to do with its meal and wine offering; with what songs to accompany it; what duties of holiness, purity, anointment, clothing, and demeanour the priests had to observe; how, when and where they should eat the holy meat, and other matters which it would lead us too far to commemorate?
The Gemara is teaching us something interesting (Sukkah 20a):
The Gemara notes: And Reish Lakish follows his line of reasoning stated elsewhere, as Reish Lakish said: I am the atonement for Rabbi Ḥiyya and his sons, as initially, when some of the Torah laws were forgotten from the Jewish people in Eretz Yisrael, Ezra ascended from Babylonia and reestablished the forgotten laws. Parts of the Torah were again forgotten in Eretz Yisrael, and Hillel the Babylonian ascended and reestablished the forgotten sections. When parts of the Torah were again forgotten in Eretz Yisrael, Rabbi Ḥiyya and his sons ascended and reestablished the forgotten sections. This expression of deference toward Rabbi Ḥiyya introduces the halakha that Reish Lakish is citing in his name. And so said Rabbi Ḥiyya and his sons: Rabbi Dosa and the Rabbis did not disagree concerning the soft mats of Usha,
However, in footnote 16 in the commentary of Rashi to Yehoshua 15:15 (linked to Temurah 16a), it says:
A total of three thousand laws were forgotten during this period. Rav Avahu said, “Nevertheless they were brought back by Osniel, son of Kenaz through his dialectic scholarship, as it is said, “And Osniel, son of Kenaz, conquered it (Kiryas Seifer).” After Caleiv conquered Chevron he approached the scholars in the Beis Midrash who were called יוֹשְׁבֵי דְבִיר, those who sit in Devir, because they sat all day studying דִבְרֵי תוֹרָה, the words of Torah, and challenged them to rediscover the forgotten laws. He announced that the one who would overcome, וּלְכָדָה (conquer) these problems would be worthy to become his son-in-law.
According to this, there were some forgotten laws, but they were "re-learned". How? The Einei Shmuel brings the Hafla'ah to the Gemara in Makos 10a and says (brought by Rabbi Mordecai Kornfeld from Kollel Iyun Hadaf of Yerushalayim):
The Hafla'ah explains that every Jew has the ability to bring a unique part of Torah into the world through his learning. When a Rebbi teaches students, he becomes a "pipeline" through which this special portion of Torah is passed down to the student and subsequently brought into the world by the student. This gives the Rebbi a special merit, for he has enabled the student to bring this unique portion of Torah into the world. In this way, his own eyes are enlightened and he becomes "enriched."
See also here
So, to quote "When a Rebbi teaches students, he becomes a "pipeline" through which this special portion of Torah is passed down to the student and subsequently brought into the world by the student"- even if there were 3.000 halachos forgotten, they were re-established so to say because of the chain of tradition. When a teacher passes down something unto his student, the chain is strengthened and passed on.. Maybe, as LFE writes in his comment, the chain of Torah was forgotten, but the chain of the tradition was not. How could there be sacrifices if we would not know how to perform them according to the rabbinical interpretation?