"For the record, I tried waiting for the computer to shut down. I
came back after an hour and it was still frozen so that doesn't
seem to be the answer".
After clicking START and then SHUT DOWN, you are presented with the small SHUT DOWN WINDOW with options to SHUT DOWN, RESTART, etc - the additional waiting I suggested in my previous posting is to be done between the first SHUT DOWN and 2nd (and final) SHUT DOWN click. I do hope this helps, as I know too well the frustration of shutdown freezes.