Welcome to Lemmy! I haven't actually heard of this issue before, I generally recommend not using offline mode though. My personal experience is that most games run fine in online mode without internet.
For fixes, I'd try going to power > change account, and see if it will let you log in again. You might want to try an ethernet connection as well if you have a dock handy.
You might also try swapping to the previous steam OS partition, you can do this by holding the QAM button (". . ." button) while you turn on the device, and selecting the second option.
If you end up needing a reset, you can try just reinstalling steam OS first without wiping your user data. That's an option when using a recovery drive.