I've got a 1983 200D that was running fine a few days ago. I swapped out the glow plug relay and it wouldn't start again. Neither would it start when I put the bad relay back in for kicks. The starter tries and tries, but the engine won't fire up. What can I do?

P.S. this car has some issue with sensing the fuel level. It stalled out on the highway with a quarter tank left and wouldn't start again. I added one gallon of diesel and it started. Immediately filled up and confirmed that it had over 1/4 tank remaining.