You can turn the water off, remove the stem and run it down to the hardware store for replacement.
However, if you open the valve, and no water comes out, it could be old rusted galvanized pipes that are the problem.
Anytime galvanized meets brass, the steel pipe loses the battle. It can close up where the two meet.
1975 is right about when plumbers were bouncing between copper and galvanized pipes depending on the cost of copper.