Help! Truck will not crank or start!

My truck was running fine and had given me no hints that it was having any trouble starting. My husband and I took my kids to the library for about an hour came out and the truck would not crank, start, or make a sound. However, all the lights on the dash we're on the radio and power was working fine and nothing dimmed when we attempted to crank it. I had a friend who is very experienced in auto mechanics to check it out and he removed the starter and took it to Auto Zone for testing and it works fine. So he put the starter back on and checked fuses, the battery, anything else he could think of and he can't figure out why It will not start. We just paid $800 to have the water pump and vent solenoid replaced a few months ago and we don't need another bill like that. We still pay on it and people were thinking that the box that disables the car if you don't pay has malfunctioned. We talked to the finance company and they say that it is enabled and have sent signals to it, but that didn't help. WE ARE CLUELESS! PLEASE HELP!