Try starting it up in "Safe Mode". If it will load fully, then there is likely something on the device that is the cause. Bad app, virus, malware, etc. You will have to remove the problem and then restart the tablet (turn it off and back on) which should let you into it again.
If it won't even load in "Safe Mode" then you may have to "Hard Reset" the tablet. But I would save that as a last resort, as it will wipe all data.
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.