Nikon F Photomic question

Hello all:
I've owned this F Photomic since 1969 (!) and it's held up very well
until recently. My son used the camera and since then this situation
has arised:
Viewing through the finder is normal, but when you wind the film, the
mirror flips into the "up" position. Upon releasing the shutter, the
mirror returns down. I'll admit that I haven't used this camera in
several years....have I forgotton something or did something just go
south? Everything seems clean, smooth, etc.
TIA
Bill.