When trying to do a "Hard Reset", which starts with the power and volume buttons, there are a few other things you need to do also. However, lets see if it will start first before trying that. I say that because a "Hard Reset" will wipe your data.
First I suggest you try starting it in "Safe Mode", this way you can see if it will actually boot up. If it does, then it is probably an app, or update to an app, that is causing your system to hang during booting up.
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 safe mode, simply repeat the steps above, and you should see "Safe Mode: Off" rather than On.
If it allows you to get into the sytem using safe mode, then you need to remove the offending apps/software. Anything you recently installed or updated could be the culpret. You would also want to check your apps list for anything newly installed, that you didn't install yourself.
Once you have removed the app/update, then try restarting the phone again, to see if it will load. If it does, then you are good. If it doesn't, then there is more on the system that needs to be removed. Repeat this until it allows you to boot completely out of "Safe Mode".
If it won't boot even in "Safe Mode" or it won't start no matter how many apps you have removed, then I would try the "Hard Reset". But only as a last resort.
NOTE: Doing this will clear 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. Press the "Power" and the "Volume Up" buttons, together, until you see the start-up logo on the screen.
3. Once you can see the logo, release the buttons and allow the device to enter the "System Recovery Mode".
4. Next use "Volume Down" button to get to the "wipe data/factory reset" option When in recovery mode the "Volume Up" and "Volume Down" buttons are how to can navigate and highlight what option you need.
5. Once you have selected the "wipe data/factory reset", press the "Power" button to select that option.
6. On the following screen, repeatedly press the "Volume Down" button until the "Yes - delete all user data" option is what is selected.
7. Then press the "Power" button to hard reset the tablet.