Try starting up the tablet in "Safe Mode" and see if it will load. If it does, while access will be limited, it will also tell you that there is something on the tablet that shouldn't be. Bad app, bad update, malware, virus, etc. You would have to remove whatever it is first and then restart (turn off and back on) the tablet.
If starting in "Safe Mode" doesn't work then either it is a hardware issue, or you have to "Hard Reset" the device, which would lose the data. Note, even if it is a hardware issue, you could lose data when it was fixed.
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...