Restart issue - HELP PLEASE

hold the power button down until it reboots or powers off.

 

Try restarting your phone into its Recovery Mode and select the 'wipe cache partition' option. Instructions on how to get into Recovery here:
https://www.hardreset.info/devices/samsung/samsung-galaxy-s20-sd865/recovery-mode/
Note that Recovery is text-only mode (use the indicated buttons to navigate through the various options), and that wiping the system cache partition won't affect your files/data. But be careful, the factory reset option is listed just above the wipe cache partition option so pay attention to what you select to do as a factory reset will wipe all your files/data.
Since you are seeing that Lock Screen message pop up though, you might also want to try starting your phone up into its Safe Mode and see if your phone will boot up without boot looping:
https://www.hardreset.info/devices/samsung/samsung-galaxy-s20-sd865/safe-mode/
It will be trickier to get into in one of these other startup modes since they involve pressing the right buttons in sequence. Timing can be a factor too so don't be surprised if it takes a few attempts until you get accustomed to what to do.