So first things first. Have you resolved the issue with the wiring at the clutch switch? If you have, have you checked the circuit to make sure that when you turn the key to the start position you are getting power to the starter solenoid?
If you have a functional circuit, and the starter is not spinning, try another starter. If the starter spins but does not disengage, remove the starter and check the Bendix drive. If the Bendix drive doesn't move the gear forward, replace the Bendix drive or the starter, if it does work, then check that the starter is installed in the correct location. You can also check the engagement distance from the mounting flange on the starter to the extended position of the Bendix drive and then compare the depth of the flywheel ring gear to the mounting face on the transmission. If all that checks out, circuit is good, starter is doing what it is supposed to, then you've got something mechanical related to the parts as they were installed.
If the shop changed the flywheel completely, or replaced your ring gear if it was worn, this could be an issue if the wrong ring gear or flywheel was installed. Some times it is possible to get gears of the same diameter with different tooth counts and they will not allow the starter to engage as a result.
Final thing to check would be that the engine will turn over. Sounds crazy, but I've seen guys use the wrong hardware on flywheels, clutches, bell housings, starters, and end up with a bolt that locks up the whole engine.
If it ran before, it will run again, ask about the stuff I mentioned, or check it yourself, but there is an answer in there.
The Current Truck (Oct-2013 to Present):
1999 (Early) F-350 Lariat, CCLB, SRW, Black/Tan
7.3L Powerstroke, 4R100, 4x4, 3.73, 37x13.50-18s, '05 Axle Swap, 6" Fabtech lift, GTP38 "D66" turbo upgrade, SCT 4-position chip with custom tuning by me, Straight Piped 4" down pipe through 5" dual stacks, N-Fab full length triple steps, Pro-Tech cab guard and tool box, louvered 5th wheel gate, Hella 500 Fog and Driving lamps, much more to come...
The one I'll miss, (Mar-2009 to Jan-2014):
2000 Excursion XLT 4x4
6.8L V-10, 4R100, 4.30, 285/75-18, 271whp / 388wtq
The one I won't, (Jul-2005 to Sep-2012):
2005 F-350 XLT, CCLB, SRW, 4x4 FX4
6.0L PSD, ZF-6S650, 3.73, 35x12.5-18, 4.5" Lift