Under US law, between 1925 and 1964, it was necessary to renew copyright every 28 years.[1] There are many works which have fallen out of copyright for nonrenewal, and there are projects which are now going through copyright renewal records to determine which of these are in fact now public domain in the US. Roughly 3/4 of
potentially copyrighted works (published since 1924 and initially registered) have proved to be public domain.
Once copyright has lapsed, it cannot be reinstated.
https://www.crummy.com/2019/07/22/0
https://www.nypl.org/blog/2019/05/31/us-copyright-history-19...
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Notes:
1. Technically, the obligation existed until the 1976 act, at which time copyright status was automatic, but was retroactively waived for works never having lapsed, back to 1964, in the 1992 act. It's complicated. (NYPL link above.)