Not enough information....did the machine go from perfectly working to completely dead, then I would suspect the battery.....did he happen by chance to try to jump start the ATV with a car or truck? I've read about it before about blowing out something in the electrical system by jump starting with something that is putting out high current...otherwise, looks like everyone above covered about anything that could happen....

 

Thanks to all who replied. I installed a new battery and checked the alternater output. It was ok. Most of the bulbs have now been replaced and the quad is working ok. Thanks again !