The latch mechanism inside the door has probably become corroded, which will jam it up so it's almost impossible to turn the lock. And it can get worse, so that's impossible to open the door. (All my 240s have doen this at one time or another.)

You can remove the door panel and lube the latch and lock mechanism. Be prepared for oil to dribble from the bottom of the door.

Check the door swich in the inner (front) door jamb of the driver's door. It sounds as if it's bad. This switch has two sets of contacts -- one for the light, one for the switch chime thingy. You can sometimes repair these switches (I've done a bunch with about a 65% success rate). Otherwise, replace it.
--
Don Foster (near Cape Cod, MA)