I had this problem on 2 machines in the middle of the day for the 6.6.7 update.
(2 x I3 10100f machine with amd 2100 pro gpus)
I went with the “chroot” procedure and it didn’t “unblock” them.
I was able to repair both of them because of this contribution from a knowledgeable forum member:
It might be a possibility for your computer. No cost to go and check if you have that option present and disabled 
Pesky bios with “CSM” are now hunted on the machines.
Best Regards 