Try turning it off and then starting it up in "Safe Mode". While it will be restricted (many apps won't work) you should be able to log in with your password. If it works then, there is probably something on the device that is the cause. Bad app, virus, malware, etc., that you need to remove.
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.