First make sure it is charged. If you have not already done so, put it on the charger for at least two hours. Then disconnect the charger and hold the power button down for at least 30 seconds. If you've done that and it doesn't restart then yes, it's dead.
Try connecting it to the usb port of a computer (computer must be switched on and set to NOT go to sleep) and leave it for a few hours/overnight. If this doesn't work you can try the 30 second trick a few times as it doesn't always wor first time.