Try starting up the tablet in "Safe Mode" and see if it will start up fully. While it will have limited access in that mode, you will also know that there is something on the device causing your problem. Bad app, bad update, virus, malware, etc. You would have to find and remove the problem to run the device normally. Now if it still won't even start up in "Safe Mode" then it could need a full "Hard Reset" or even be a hardware issue.
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.