I once had a similar problem with my X-700. After much hair pulling, I discovered that the mirror had become caught, which prevented me from firing the shutter. This camera uses a thin, ring shaped, black plastic sheet in the lens mount mechanism. Not sure what it is supposed to do, but it had become bent and dislodged when I was changing a lense. Next time I fired the shutter, the mirror got caught on this thing. I was able straighted it out a bit and push it back into position and that solved the problem.