Hi,
First try a ''soft reset'' on your tablet. Here is a link that shows you how to do it.
[guide|27634]
If that doesn't work you will have to'' hard reset'' your tablet.
'''Be aware that a hard reset will erase all your downloaded apps and user data'''
Power off the tablet.
Press and hold the Volume up button, which is located closest to the touch screen lock switch.
With the Volume up button held, press and hold the Power button for 3 seconds then release it. Continue holding the Volume up button.
Once the tablet screen turns on, release the Volume up button.
On the Select Boot Mode screen, select [SD Image Update Mode]. Use the Volume up and Volume down keys to navigate through the menus and use the Power button to select the desired option.
On the Android system recovery screen, select wipe data/factory reset.
When prompted to Confirm wipe of all user data, select Yes -- delete all user data.
On the Android system recovery screen, select reboot system now. Your data will be erased.
Your tablet will reboot and go to the Welcome screen. Follow the onscreen steps to setup your tablet.