Try starting up in "Safe Mode". This should let it start up without access to most any app (including whatever one is loading). Then you can go and remove whatever is the cause. Bad app, virus, malware, etc. Then restart the phone (turn it off and back on) and it should work fine.
Should this not fix it, then you may have to consider a "Hard Reset". However, that should be a last resort, as it will wipe everything.
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.