Rear camera is not working

@bagban you did not tell us if you get a message on your screen or not. The first thing I suggest is that you start your phone in safe mode first. Here is the link to the Samsung site that tells you how to get in safe mode. Once there, check to see if your camera still acts like it has failed.

Next you want to try and clear the cache Settings ==> Apps ==> Camera== > Force Stop ==> Storage ==> Clear Cache ==> Clear Data. If you are still having issues, check the circuitry/main board according to Samsungs flowchart

To find the component required to measure, your this document Component Layout and with the search function of your PDF reader, find each one of those on your circuit board.

To work on your camera use this guide Samsung Galaxy S21 Ultra Rear Camera Replacement