Take it back or take it to a dealer with repair facilities.
I have a Nikkormat FTN and a 50mm f1.4 lens and, as it's a fully manual camera, you should be able to change the aperture and shutter speed to any combination you want - even if the end result would be massive over or under exposure. The fact that the lens won't come off sounds quite odd / worrying.
They're great cameras, built like tanks and should (hopefully) be capable of being repaired so long as you don't damage it by yanking / pulling / twisting stuff that's stuck.
Hope it works out for you.