I've lost access to my encrypted files

Solution 1:

Unfortunately, Windows is doing its job by denying you access with your old key :/ Windows requires a new key with a new password when it encrypts so someone with an old key gets locked out, just like a dead bolt.

Solution 2:

I misread your question - missing completely on that you had ABRUPT loss of power. This could mean any variation of possibilities, but corrupt file may be one of the big reason. This is exactly it happened to me when an encrypted excel file could not be rescued after a hard drive crash.

Have you give a try to password remover for your file type? (e.g.Elcomsoft's Word Password Remover, Excel ....?) Cost of password remover offsets the value of the data file, IMO.

Running out of ideas.