It can't be anything too major or it would have never cranked in the first place. Did you check simple stupid things like it being in neutral, all the fuses good and connections to the fuse box, battery etc good? Does it crank when you jump the poles across the starter solenoid? It should do that with the key out. If it does then your issue is in the main ignition switch, the starter button, the solenoid itself or any of the safety interlocks (clutch, neutral, tip over, kick stand). If it won't crank when you jump the solenoid then its either bad starter, low battery or bad connections.