If your body is afflicted with this fault, you will see nothing but absolute blackness upon looking into your viewfinder (even with the lens cap removed), yet your film transport will continue to advance as usual when cocked and fired. In this condition, the camera will still take photographs, however you will not be able to frame, focus or meter the shot you wish to take. In other words, the camera is of no constructive/creative use to you. If sounds like your camera, this tutorial is here to help you.
Contrary to what some messageboards will advise, this fault is not electronic and is not the result of a died battery or failed circuitry. This is true on more advanced Pentax bodies, but does not apply to the Spotmatic series.
Before attempting this particular repair, check the foam "bumper" that cushions the mirror during exposure. It may have become sticky with age and just needs to be cleaned. In some instances, cleaning the top of the mirror with some rubbing alcohol or brake cleaning fluid, and dabbing a teeny-tiny bit of graphite lubricant across the foam with address this. However, this particular method will only work with mirrors that become seized every so often, particularly when shooting at slower speeds. If sounds like your camera, do this first.
Also, this repair method will not help you if your mirror has become seized, meaning that it is stuck mid-swing and/or will not move if even you try to force it by hand. Thanks to Kiteman for bringing this to my attention.