Sounds like either the whole thing needs resetting ("Hard Reset") or there is just something on the device (bad app, virus, malware, etc.) that needs to be removed. Try starting it up in "Safe Mode" and see if it will load. If it does, then you have something installed that needs to be removed. If it still won't load in "Safe Mode", then you may need to do the "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 the steps above, and you should see "Safe Mode: OFF" rather than on.