There are about half-a-dozen volunteers here, spread out over 8 time zones. We do what we can.
It seems that Audacity thinks you are trying to open AAC (Advanced Audio Codec) files, which require ffMPEG. Your old files didn’t suddenly change to AAC files. Something else has changed.
First, please try resetting Audacity by following these instructions: https://forum.audacityteam.org/t/resetting-audacity/50613/1 but only delete the audacity.cfg file, not the plugin files. DO NOTE DELETE pluginregistry.cfg or pluginsettings.cfg.
If that does not work …
Please tell us exactly what file(s) you are trying to open. The full file name, including extension. How are you trying to open it: with File > Open or File > Import Audio?
– Bill