SSD is locked

Windows 10 using a 240 GB SSD Plus:

Due to failure to load Windows I have used my .iso disc to attempt a recovery/repair.

I can bring up a screen titled “Reset This PC”

“This drive where Windows is installed is locked. Unlock this drive and try again.”

How do I unlock the drive?

I have looked at 20+ forum links and none of the instructions presented have worked.

It seems that the problem may be a corrupted MBR and I am unable to get Windows commands to find the drive letter–even though it is C:  and the only drive in the computer.