The collar is #20 (EFLH1214) and the bearing is #27 (EFLH1215) on the exploded view in the back of the manual. Having the collar slipped up the shaft like that could definitely cause the shaft to slide up and down, engaging and disengaging the gear; the collar's job is to keep that from happening. My guess is that the top rotor head was jammed against something during the crash.

The collar is still there (the upper of the two metal rings between the frame and swashplate in your photo; the lower ring is the bearing), so you shouldn't need to replace any parts, nor should you have to disassemble anything else: just loosen the set screw on the collar, slide it down the shaft -- pushing the bearing into the frame's tube, making sure that the gear on the bottom of the shaft is resting against the lower bearing and meshed with the motor gear -- and retighten the screw. To reiterate what I said in my previous reply, the gear on the bottom and the collar on the top should be barely touching the respective bearings so that the shaft doesn't bind up.

Edit: Corrected the reference numbers and part numbers.