Hi Ron Thanks for all these suggestions. I'm afraid none of them showed up any issues. I tried SFC, DISM, BIOS Recovery...nothing worked so I decided in the end it was going to be quicker and easier to rebuild the whole system from the ground up rather than try to diagnose and cure the problem. All of my data is on external drives so I just flattened the system drive and started again. It is now running version 22H2 which it was incapable of upgrading to from the 21H1 version that was preciously installed. So everything is now hunky dory, thanks. Alan
I have an Inspiron 5490 All in One  and last night I updated the BIOS from ver 1.20 to 1.22 as it was shown as a critical update. After doing so the computer booted but Windows Explorer will not longer run. I have a blank screen unless I call up the task manager from where I can manually run some programs. If I try to start explorer.exe, it appears in the task manager list for a few seconds and then disappears. I have tried to downgrade and roll back the BIOS as there is a setting in the BIOS that is supposed to enable me to do this but it refuses to downgrade and it tells me that operation is blocked. Dell have effectively bricked my PC with a bad BIOS update but they will  not help because my machine is out of warranty even though they caused the problem and they are preventing me from downgrading to a BIOS version that worked perfectly by applying some stupid rule because it was a security advisory from Intel. By applying that rule, Dell has done just as much damage to my PC as any hacker could by exploiting whatever security hole there was!
BIOS Ver. 1.22 problem