My sons oset just stopped moving all of a sudden. All lights are on the handlebars, but it just isn't moving. I've checked the wires and they all seem to be connected properly.. do i suspect the worst in that the motor is broken or could it be something simple?

could be something simple,, check throttle, ideally swap out if you dont know how to test same with speed pot (dial), check all connections ( throttle and speed dial ), 

Thanks for the reply. I'll get someone to use a voltmeter on it thanks