My 1958 Nash Metropolitan has been running great, but all of a sudden, she won't shift into gear. What's up??

The clutch is likely shot. Drop the transmission and install a new clutch, pressure plate, release bearing, and pilot bearing. I'm sure you have a clutch that can no longer be adjusted. Not too bad of a job for a serious do-it-yourselfer.

Yup sounds like a clutch to me too.