You will have to reset the tablet to its factory settings. Unfortunately this will lose all your data on the tablet.
Make sure the tablet is fully powered off and you have not simply turned off the screen. With the screen showing the screen for your login pattern, hold the power button pressed for about 20 seconds until it powers off.
Try holding the volume down button pressed while you press the power button. Your tablet might be different, but try it. If this is the right combination, you should see a menu of options appear, one of which should be to restore to the factory settings.
Use the volume up and down buttons to select the menu item you want, then press the power button. Make sure that the battery is fully charged before you start.
I hope this solves your problem.