Help! Why won't my truck start?

I have a 1992 Ford Explorer, and it isn’t working. I have checked all of the fuses, and wiring. The starter and the fuel pump seem to be working fine. The battery works, but it will not start. My father had the transmission replaced, and the starter and solenoid were replaced a little after that. At first, you had to have it in reverse to get it started. But now, it won’t even do that. I don’t know if it could be the tranny, or an electrical problem. Please help.