My K-50 acted up in a similar way today. I got mine in Jan, 2014, so much for warranty! Anyway, my camera would power up and mirror was stuck in the up position. After trying everything (camera reset, dust removal to vibrate it, remove / install lens & card & battery etc, etc, etc...), it finally would take photo, but photo came out black, with just a hint of an object (and this is with flash on & automatic & manual trials). Ten minutes later power on, power off, repeat tinkering, it finally took a normal photo, noting seemed wrong. Took it home, same thing...camera snapped photo & mirror was NOT stuck up....photo dark. I tried putting my AA adapter in with fully charged NimH batteries, and it worked fine. I'm HOPING it was just a weak battery, although at the 1st failure, my Li-Ion battery was 3/4 charged, so...GULP! I think it's gonna have to be repaired, but I'm still hopeful it was only weak battery.
Gonna try reinstalling firmware for it tomorrow, once batteries have fully charged.