Need Help diagnosing problem.

[CAMAROSS]

Went to get my pickup today after 4 mons at the paint and body shop. Looks great! Problem is my friend that did the work said that he went to crank it one day to move it in and it wouldnt crank so they had been pushing it in and out. I got there and it fired right up. Great! Next day, my wife carried me to get it and it would not crank for anything. 1987 Silverado. fuel injected TBI etc. I checked at the relay and it was getting fire going to the relay so I got under it and unplugged the harness going to the fuel pump and no fire there. Cant find any loose wires. I tapped on the fuel pump to no avail. Whats up with this????????