2003 ford f-250 will not start

Try putting vehicle in neutral and starting it and see what happens. I am thinking it may be your neutral safety switch.

good idea, but, unfortunately, it will not start in neutral either, does the same thing

Good thinking, I would try to verify if I lost voltage to the switch, or from the switch. The trouble with intermittent electrical issues can be access to all the places power goes, connects and continues, the connectors, the controls in the circuit. after 10 years, you have to consider the vehicles usage, does it take a pounding, run hard tires, had power distribution box work done, or issues in that power supply center? are the firewall connectors getting a jarring or pounding, or the fuse panel? some connectors are subject to loose fit after 10 years, the wiggle test we started using in the 70's 80's and after can speed up the process. Or a test light or voltmeter connected where it should have power but doesn't, then rap on things with a screwdriver handle, we affectionately called that the hammer test. Every Ford tech as 3 main tools the BFH , The mFH, and The screwdriver. Then it gets technical.

Ground circuits can be verified from the battery, the body,the engine, and the frame, and each to the other, no continuity readings exceeding .5ohm are left that way, clean, tighten, or replace to establish good ground.