Sounds like the winder gear has gone bad and striped out, the 645 model is a great camera and I am sure it would be worth the repair, the company I have used for all of my repairs over the last few years has been Kelly Camera in AZ, you might take a look at their webpage and drop them a note www.kellycamera.com

the reason I like them is they have the ability to make parts in house for stuff that is not available anylonger.

Good luck.

Dave