There was a vulnerability attack on certain Lemmy instances. As part of the resolution to the attack, all accounts have been automatically signed out. The client currently does not handle this gracefully so you will need to manually sign out and sign back in. I am working on a fix for this.
And enable 2FA if you can. Sure, it's annoying to have to dig out your phone (or browser extension, desktop app, etc.) for a code, but it's better than having your account taken over.
I could be wrong but I think the attack circumvents this. As the attacker would receive a users JWT token the 2FA in the login process is moot. Still better to have it enabled in general tho.
All JWT tokens should have been invalidated by the server creators. Those tokens should no longer be usable. It's still possible actions were taken using your account while the server was compromised. See https://lemmy.world/post/1290412