Sounds like Lemmy was hacked somehow last night, and their fix for it involved invalidating everyone’s user credentials, so you’ve basically gotta log out and log back in.

I don’t think there’s a way to log out in Memmy, so what worked for me was going to accounts -> change account settings -> edit account and then just clicking “save” without actually editing anything. I’ve seen several others say it worked for them too, so maybe give that a try.