Well first you need to get it to turn off. If it won't on its own, you will have to wait until the battery runs out of power.
Once it has run out of power, plug it in to charge but leave it turned off.
Then after it is fully charged try starting it up in "Safe Mode" and see if it loads. If it does, then you most likely have something on your tablet (malware, virus, bad app, etc.,) that is the cause. Or a recent update to anything on the tablet. You would need to remove, uninstall, clean (for virus, etc.,) whatever is the problem and then reboot the tablet.
If that doesn't work, or it won't start in "Safe Mode" you may have to do a "Hard Reset", but that you should really save for a last resort as it will wipe all your personal data...