Your narrative is not easy to follow. Timing belts like to slide off? Not if they are installed correctly. Bumping the motor over with the starter is not going to do any less damage to valves than starting it up. If you want to turn the motor over carefully, it needs to be done by hand with a wrench on the crank/flywheel.

Since it is not firing up, did you reconnect your coil? What else did you disconnect during your timing belt swap procedure? Have your double checked that you reconnected everything? Do you have any Check Engine Light codes? If you have a sensor issue or forgot to plug something in there will be a code present. Compression seems good mean absolutely nothing unless you actually used a compression tester and have hard values to give.

Report back, otherwise shooting in the dark is not worth your time.

---

2012 2500HD LML