If there is no movement when you try to swipe to unlock it then your digitizer(the glass that senses touch and is separate from the LCD) has either been bumped and the ribbon cables that attach to the board could have come loose or the unit has gotten wet and shorted out the board.
If you CAN move to the unlock but it doesn't then try re-starting the tablet by holding the power button down for 10 seconds. Wait about a minute then turn it back on and see if it has corrected itself.
Apps running in the background can cause this to happen.
Answers & Comments
If there is no movement when you try to swipe to unlock it then your digitizer(the glass that senses touch and is separate from the LCD) has either been bumped and the ribbon cables that attach to the board could have come loose or the unit has gotten wet and shorted out the board.
If you CAN move to the unlock but it doesn't then try re-starting the tablet by holding the power button down for 10 seconds. Wait about a minute then turn it back on and see if it has corrected itself.
Apps running in the background can cause this to happen.
Hope this helps.