@Hislam: I'm sorry to hear that you have encountered this issue. Is the black screen persistent, or have you been able to restore access to the phone? If the screen has remained black, can you try pressing the Volume Down and Power/Side Keys together for around 10 seconds to see if you can perform a Soft Reset, and address any temporary software issues that might be a contributing factor?
@Hislam: What steps did you take to restore your screen? I should point out that your Galaxy S21 is designed to enter an idle state after 30 seconds of inactivity, therefore I recommend tapping the Power/Side Key next time it the issue occurs, just in case the above is applicable to your situation. If the screen goes black, and the touch screen and hard keys become unresponsive, then you may wish to restart the phone in Safe Mode, and monitor the performance for a set period, depending on the frequency of the issue. This will disable all third party apps, and help to determine if the problem is stemming from and external source.
To enter Safe Mode, press and hold the Power button until the 'Power off' prompt appears on the screen then release > Touch and hold Power off until the 'Safe mode' prompt appears then release > To confirm, tap Safe mode. If the problem isn't present in Safe Mode, then restart the phone as you would normally, and delete any third party applications.