Having same problem. System had been rock solid stable with Win10 version 2004, installed 20H2 update and the next morning I walked into my machine having completely locked up. Nothing helpful in the system logs to indicate the cause. I let it go and 2 weeks later, same thing. Hard locked while the system was idle overnight - nothing in the system logs. I just rolled back to version 2004 today, so we'll see if that fixes the issue or not.
Just providing an update here, SO FAR, it's been stable again since rolling back to version 2004. Now it's only been like 12 days so that's not a huge sample size, but no signs of any issues since the rollback. I'd recommend some other folks try rolling back to 2004 also and see if the issue goes away.
Just another update here as it's been another 12 days (24 in total) since I rolled back to version 2004 - zero crashes or hangups since, so I will absolutely avoid the 20H2 update moving forward. May even skip the 21H1 update as well...Microsoft really needs to work on their QA.