It's because your graphics driver, probably from the oibaf:graphics-drivers PPA got a faulty update, that you need to undo somehow. There was an onpour of these issues here on AskUbuntu last week. Some of the questions got apparently working answers.
– LeventeApr 22 '21 at 21:27
I don't know for sure because I did not have this issue. But here are two where an answer at least got an upvote: https://askubuntu.com/q/1328152/1157519 and https://askubuntu.com/q/1329451/1157519
– LeventeApr 23 '21 at 08:12