Try starting it up in "Safe Mode" and see if it will load. If not, then you may have to "Hard Reset" the device. If it will load however, it will have limited abilities in that mode and you will have to then scan your device for viruses, malware, bad apps, etc.
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.