Well, it could be that it isn't charging. Have you checked to see if it is? If it isn't, then the problem could be the charger cord and/or the battery. I would check the cord first, as the is the easiest one to check. Try using a different cord on the tablet and try testing your cord on a different device.
If that isn't the problem, you may need the battery replaced. In this case, if the tablet is still under warranty, then you should contact the manufacturer about a repair/replacement. You don't want to do anything that would void that warranty.