First try restarting (turning it off and then back on) the tablet. Often this will fix the problem. However, should that not do it, then you can try...
1. Go to "Settings" and then "Apps".
2. In "Apps" go to "All".
3. Locate and tap to open the "System UI" app.
4. Tap "Clear Cache" and then restart (turn it off and back on) the tablet.
Should this still not fix it, then you are likely looking at at full "Hard Reset".
NOTE: Doing this will clear 'all' the data on your tablet, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the tablet...
1...