The first thing you need to do is to power the phone. If it doesn't begin to work, remove the battery for a few minutes then plug in back. Repeat the step and if you encounter the same issue, then you'll have to send it to service repair. You can take advantage of your warranty (if it's still available) or visit a local phone service shop. They should give you additional details about the issue with the phone. Do not do the factory reset because you might lose everything.