It will be interesting to watch the anti-ebike crowd go into a feeding frenzy with thiis one "oh look an ebike had a mechanical... conventional bikes never have mechanicals... bla bla bla..." The last failures I have had to deal with on conventional bikes (both mine and a friends, on different days) that downed the biks on the raod would have been still ridible if they had been ebikes (broken chains, both times).
Mechanical things break. I will argue that the answer to
Hmmm. Would you have been able to ride it home if it was a normal bike?
was most certainly a yes. for that matter I expect that the rider was able to ride the ebike home; or across the county, for that matter, the rider CHOSE a different option.
Enjoy your feeding frenzy, all the while pretending that have never had, and will never have, a mechaincal failure of a system that DOES NOT leave the bike unridable.
On to the problem, unless there is a good reason not to, you cheould be able to test the controller by bypasing it. Run, using test leads, power straight from the battery to the motor. If the motor spins up (and be ready, it will go to full speed) then the motor is probably fine.