Try starting it up in "Safe Mode", while it would be restricted (not all apps work) if it loads fully in this mode, then you know the problem is something along the lines of a bad app, virus, malware, etc. To resolve that you would need to remove the problem and then restart the phone.
If, however, it won't load in "Safe Mode" either, then you may need to do a "Hard Reset".
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...