Try letting it drain completely (or if not under warranty anymore you can try opening it up and pulling the battery. Yes I know that is complicated on most tablets.) Then once drained charge it for a short while (not fully) but don't turn it on normally. Try starting it up in "Safe Mode" and see if it will load completely. If it does, it will have limited access, however this will tell you that there is something (app, virus, malware, etc.,) on the device that needs to be removed to get it to work correctly. Once removed, then restart the tablet and it should load fine.
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.