Something may be jammed in the door latch mechanism that's preventing the lock from turning. You can remove the lock cylinder fairly easily by pulling the flat metal clip at the rear of the door edge. If the key turns once you have the cylinder out then the door latch mechanism is at fault. you'll probably need to remove the latch to check that.

Sagging door handles are a common problem. Most of the vendors sell replacement springs as a fix.


Kevin
1951 Chevy 3100 work truck
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1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car)
Busting rust since the mid-60's
If you're smart enough to take it apart, you darn well better be smart enough to put it back together.