It sounds like you have something on the table that is preventing it from turning off. Bad app, malware, virus, etc. Try turning on the tablet in "Safe Mode" and see if it will then let you turn it off. If it does, then you need to scan the tablet for the problem and remove it.
Should this all not help, you may have to do a "Hard Reset" on the tablet.
To get into "Safe Mode" please do the following.
1. First turn off the tablet.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.
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. Make sure that the tablet is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
5. Next use "Volume Down" button to get to the "wipe data/factory reset" option When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
6. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
7. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
8. Then press the "Power" button to hard reset the tablet.
9. If you took your SD card out in step 2, when the device is up and running again, reinsert the SD card.