If we cut to the quick:
If there was nothing wrong with the gearbox, then the only conceivable way I can imagine to end up with the rotors stopped in flight is:
Engines were stopped (Either flamed out or switched off)
AND Collective was too high (Either pulled up inadvertently or deliberately, or there was something (foreign object?) preventing lowering.)
AND MAYBE rotor brake could have been applied
There is little further speculation to be done. We can only wait to find out which of the above options happened.