Couple of years ago, there have been multiple threads about this issue, and also a way to fix it, or at least to try it.
It is about the same issue for the ME, ME Super, Super A, Program A, P30, and P50.
As far as I remember, there is a mechanical hook which does not properly lock anymore, caused by either dirt/old lubricant, or wear, or a combination of both.
I think it can be seen after removing the bottom plate, but access seems not to be so easy. So it's a classical case for CLA.
One of my ME Supers shows the same problem. With this, I can cure it by heavily, near to destructively working with the cocking/transport lever. But with this body, the problem always returns if cocking and releasing is no done several times a day.
@Ray-uk
If the mirror is not stuck in the up position (which would be quite another problem), there is no way to test the mirror function.
Mirror movement is done mechanically only, the energy for it comes from cocking. No cocking --> no mirror movement.
Only with later AF models mirror movement is done by an electrical motor.
EDT:
Found one of the old threads. There you will also find more links.
https://www.pentaxforums.com/forums/8-pentax-film-slr-discussion/236675-me-su...ml#post2506926