Rear hub not worked loose

Hi all, I took my scooter for an mot yesterday, which it passed, he did say the rear brake was bottoming out which would need adjusting, however I wanted to put some miles on it as I recently put a new scootopia 185 kit, within about 10 miles I didn’t seem to have much power in fourth gear so had to keep dropping back to third which after a while worried me I thought the top end may be seizing, eventually it would hardly move, luckily outside berkhamstead golf club, struggled to push it over the chipping/putting green and put her on the stand, engine ticking over lovely by the way! The rear wheel hub nut was missing but the locking washer was still in place albeit rounded! Any ideas how this could of happened?, the threads on the lay shaft look ok!