Are there any codes? Have you checked the grounds for corrosion or being loose? Have you examined the RCM connectors for any traces of burning or corrosion? When you turn the key to the On position, do all the dash lights light up. Does the black rectangle in the instrument cluster change to a D? Does the fuel pump start for a few seconds after the key is in the ON position. Do the door locks work (this checks if the CLP/SS is working)? Does the key fob open and close the door locks (checks the SKREEM receiver) ? Has it rained lately, and have you noticed any wetness on the passenger's side carpet (the TMU is under the passenger carpet). Did you spill anything on the shifter (if the ECU doesn't know what gear you're in, won't crank)?
With a little more info, should be able to narrow it down.
Good Luck, keep us updated.
-Jerry