I think I already know the answer, but wondering if this is something simple. I am selling my 500cm and was showing it to someone today. When they tested the mirror lock-up, the camera comletely jammed. Can't wind it on, can't use the shutter, nothing. Not sure what happened. Anyone? Yes, I know I can send this in for repair, obviously. Just wondering if there is some simple thing here that I don't understand. Thanks!