I was loading a film into my Yashica Electro Professional (the original one, ASA only goes to 400.) I realized the shutter lock was on (i.e. the ring around the shutter button was rotated to the lock position) so I couldn't fire the shutter. I turned the lock to open, but this time it didn't feel quite the same. And now the shutter won't fire at all. The lock is stuck on. I've tried wiggling and jiggling the lock to no avail. Is the shutter lock getting stuck common? Is there a simple remedy? Or do I have to tear down the top plate of the camera and see what's up?