I am including the steps to "Hard Reset" below, just in case they are not the ones you tried to use. Should it still not work (or if this is what you already tried) then the problem is likely going to need to be looked at by a tech. I would contact the manufacturer if it is under warranty, or try a local tech if it isn't.
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...