A language declared extinct got its 'critically endangered' status back

Cornish stopped being anyone's everyday spoken language by around 1800; its best-known late speaker, Dolly Pentreath, died in 1777. Starting around 1904, revivalists built new grammars and dictionaries and taught it as a second language, reviving it enough that UNESCO reclassified Cornish from 'extinct' to 'critically endangered' in 2010 — one of very few languages ever formally un-declared dead.

— Wikipedia contributors, Cornish language — Revival of Cornish