Try starting it up in "Safe Mode". It will limit what you can access, but if what you can access works fine, then you know you have something on the tablet that is the cause. Bad app, bad update, virus, malware, etc. that you will need to remove before the tablet will work normally again.
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.