Why won't my JD 4115 tractor start?

My tractor had been running fine, but one time it just stopped running. I cranked it a few times and nothing happened. I got off, wiggled a few things, then it started as normal and ran fine the rest of the day. The next day I tried to start it, and it cranks fine, but the engine does not start. I guessed it wasn't getting fuel, so I replaced the fuel filter (which wasn't due to be changed) and the air filter (it was quite dusty). But it still won't start. Could it be a pump issue, something with the injectors (i don't think so, as it seemed to run fine beforehand), or possibly something else electrical? I'm just looking for things I can try to diagnose the problem.