If the device will not turn on but you have been charging the device, then the problem might be your charger.
Try using a different charger to charge the device. If the device still does not turn on then there are many possible other problems that can cause this: Some of them are fixable while others are not.
Like Grant suggested, it could be a faulty battery in which that can be replaced.
Also, it can be a software issue. In this cause the device might be "bricked" which means that the device is missing necessary files in order to boot and fails before initializing the boot up process. If this occurs then you may way want to contact the manufacturer and talk see if they can repair the device.