Welcome to Android Central! Let it charge on a wall outlet for at least 6 hours, then press and hold Power for about 20 seconds. If nothing happens, try using a different charging cable/plug. If that still doesn't work, then the battery might be failing, or there could be a problem with the Power button or the motherboard. Any of those issues would require a trip to the repair shop (or a call to HTC, if it's still under warranty).