Sounds like you had something on the device (app, virus, etc.) that has caused problems. If the device is under warranty, I would contact the manufacturer.
You can try doing a "Hard Reset" of the tablet, but do note you would lose anything on it.
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...