To do the "reset" you will have to remove the bottom plate on the camera. From there you should be able to see most of the mechanical workings that trip the shutter and mirror. The very last photo on the first page of the Kyphoto link I provided above shows the lever that resets the mirror. When I was working on my ME Super I was able to trip the lever by moving it with a little screwdriver. This should reset the mirror, but no guarantees that it will not happen again. After 20 plus years, rubber seals and gaskets start to turn to goo and things start to stick together. This is why people recommend a CLA (clean, lube, adjust) for your camera. That should set everything right if you don't want to work on it yourself.