Peter Zack , Mar 01, 2010; 08:21 p.m.
I suspect (and only a guess) that the shutter has jammed or malfunctioned. The mirror is only part of the mechanism and I know on a film camera, when the mirror is stuck up, you can stil use a small screwdriver to push it down but that doesn't solve the problem. It's just on a spring loaded hinge. The problem is that the shutter won't open or close.
So I think service is the only way to get this fixed. Shutter assemblies are fairly intricate and fragile. Without proper technical knowledge I think it would be a tough DYI fix.