Do a "wipe cache partition" This is done similar to a factory reset but instead of selecting the factory reset select the Wipe cache partition. [https://www.youtube.com/watch?v=C6zweh7mnnE|this video shows]how to do the factory reset but instead continue to scroll down until you find wipe cache partition. Its actually off screen so you need to scroll down further. If this brings no joy you will need to do a factory reset . This will cause you to lose all your data and apps and return the tablet to factory settings. Hope this helps