Lock Screen

If this pertains to user profile " iMac 21.5″ - suggest using a USB Keyboard and Mouse as the Apple Bluetooth connection may not function for the purposes below ➡️

1 - Restart in Safe Mode. This will perform a Disk Repair, clear cache files and only load Apple Software, extensions and fonts. The boot up will be slow and can take some time - Normal.

2 - Safe Mode will also eliminate Third Party Software, extensions and drivers from loading 

3 - Does the issue present in this mode ?

4 - Sometimes a Safe Boot followed by a Normal Boot will just put things right

yes I also faced the same problem with BIG SUR. after a couple of PRAMs and NVRAMs it stopped

step 1

1. Power off your Mac.
2. Press your Mac’s power button and then hold these four keys at the same time: ⌘ + option + P + R.
3. Continue holding the keys until your Mac restarts for the second time. On older Macs, the chime will sound upon restart. On newer Macs, the Apple logo will appear and then disappear.

step 2 Use Terminal

1. Close all apps and open Terminal (Finder > Utilities > Terminal).
2. Enter sudo nvram -c and press Enter/return. Enter the Administrator password and press Enter/return.
3. Enter sudo shutdown -r now and press Enter/return. The computer will now restart your computer.

Note: use terminal with caution