It could be a number of things. First you need to turn it off completely. Once it is off, then try turning it on again. If the same thing occurs, then again turn it off, but this time when you go to turn it back on instead of doing so normally try turning it on in "Safe Mode".
If it will load in "Safe Mode" then you likely have something on the device that is the problem. Malware, virus, bad app, etc. What ever it is will have to be located and removed. If you cannot find the issue, then while in "Safe Mode" you should back up all your personal stuff (photos, contacts, bookmarks, etc. Except apps, they can't be backed up) and then do a "Hard Reset" of the device.
Now if you can't get it to start up even in "Safe Mode" then you will have to try a "Hard Reset" which means, since you can't back up your stuff, everything on the tablet would be lost.
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.
To do a "Hard Reset"...
NOTE: Doing this will clear 'all' the data on your tablet, so it is suggested you not choose this option unless you have to, or aren't worried about losing anything on the device.
Be sure you back up any data you want to keep, to a computer or cloud account, before doing the reset.
To "Hard Reset" the tablet...
1. Make sure that the tablet is actually powered off.
2. Remove your SD card, if there is one in the device.
3. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
4. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
5. Next use "Volume Down" button to get to the "wipe data/factory reset" option.
6. When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
7. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
8. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
9. Then press the "Power" button to hard reset the tablet.
10. If you took your SD card out in step 2, when the device is up and running again, reinsert the SD card.