My thought process....
Check all fuses/breakers (which I'm sure you've already done).
Check that there's no loose solder connection or wire connection at your thermostat (made me think it might be a cold solder joint when you said you wiggled the fuse and the display came on). There's got to be two circuits to control two separate A/C units so the circuit that controls the non-functioning unit may have a loose wire/connection.
If all that checks out, then you'll have to climb up top and check all connections at the A/C unit itself as has already been posted. Suspicious would definitely be the start-up capacitor. Of course, there may also be a compressor disconnect switch up there somewhere too. Not that familiar with roof-top A/C units....fortunately mine have not given problems...yet.... ;-)
Good luck and let us know what you find!
