Here's a link to a site with a PDF of the manual:
Pentax MG instruction manual, user manual, free PDF manual, camera manual
But if the mirror is up, your camera is broken in some way. The only time the mirror should flip up is when the camera is taking a photo. It may be stuck up, glued by the foam light seal in front of the focus screen. Or the camera is not completing its cycle after taking a photo. The second problem sounds more likely in your case.
You might try setting the mode dial to the mechanical shutter speed setting, 1/100 with a lightning bolt. That would take all the "auto" and electrical stuff out of the loop. The camera might start working and you know there's an electrical issue. I think the problem is actually mechanical, and you can fix it by removing the bottom plate, figuring out which gears are supposed to move, and encouraging them to complete their cycle. But I don't have any photos or specific descriptions of the parts involved. There is some information here, especially about "wind lock":
Favorite Classics > Pentax ME Super Ramblings > Three Common Failure Problems
I was able to fix an MG myself using this information, I can't guarantee it'll work for you.