Which phase is shorted to positive?
And positive is shorted to what?
The more specific exact details you give, the more likely people can give exact advice.
For now, I can only recommend going to the http://ebikes.ca
site for the Learn - Troubleshooting section, and see if any of that is helpful.
If youve already done those tests, and the FETs all test bad (shorted), then youll probably have to replace them all.
As to why they failed, could be a bad controller, bad FETS (counterfeit, etc), wrong phase/hall combination causing too high a current and heating in the FETs, making them fail, bad setting somewhere in the programming, problem with motor phase wires/etc causing a FET failure, etc.