https://wccftech.com/m1-macs-screen-saver-bug/

Soon after Apple’s M1 Macs were said to receive a fix for Bluetooth issues, a screen saver bug starts affecting some users. With macOS Big Sur, the company introduced something called Fast User Switching. This feature allows users to quickly switch between accounts without logging out, saving time as a result. Unfortunately, performing this can sometimes trigger a screen saver, which shows up on the new M1 Macs.

When users attempt to dismiss the screen saver, which is done with a key tap or a trackpad click under normal circumstances, nothing happens. The mouse pointer can still be seen, and you can move it in any direction. However, nothing happens, and you’re completely locked out. To remedy the problem, M1 Mac owners have to either close the lid, open it again, press the Touch ID key, or use the Alt-Command-Q key combination to reaccess the lock screen, as explained by MacRumors.