Before doing / suggesting anything, I'd like to say sorry towards your GRF.
you may, first, want to try to open your GRF with GRFBuilder.
If it opens, then extract all of them and make sure the files are not corrupted. Don't delete them, for backup
If it said to be broken, then, that's where my sorry goes to.
I encrypted my GRF once, and it broke down. I lost all of my modification, I can't edit it and I didn't have the backup (I don't know why).
Good luck! (PM is available)