A similar problem can happen with the Minolta XM / XK. If the battery fails and the shutter is released, the mirror locks up. Apparently some Minolta XM are sold as defective for this reason. In order to free the mirror one has to activate a certain minuscule push-button. Minolta engineers did - as ever - the right thing and put the push-button in the thread of the cover of the battery compartment. When you take out the battery cover, you activate the button. The button is visible so, in case the thread does not activate it on an old camera, one can activate it with a fingernail.