Battery. I'd try charging it, or jumping it (if using a car, don't have the car running) and I bet it fires right up. A better check is a volt meter across the battery terminals. It SHOULD be 12.6 volts, or pretty close. If it's down in the 11's, it's low, might just need a charge. Any lower than that, and it's likely shot, unless the bike has been sitting for quite some time.