PSA: irc.libera.chat (home of #emacs) has started requiring SASL to connect.
https://libera.chat/guides/sasl
If you're an erc user and you have your credentials in .authinfo.gpg, adding the following should be enough to let you connect to libera.chat again.
(add-to-list 'erc-modules 'sasl)
(setq erc-sasl-user "nickname") ; Replace this with your own registered nick.
(setq erc-sasl-auth-source-function #'erc-auth-source-search)
https://www.gnu.org/software/emacs/manual/html_node/erc/SASL.html