Hi
You need to restore using your PC just follow this.
Turn your iPad off .
Make sure your PC is running the latest itunes 12.4
Connect lead to your usb on your PC .
Press & hold power button on your ipad DON,T let go
Connect lead to iPad keep power button pressed down until PC sees ipad
Then wait for restore .
You may need to do this more than once before it works.
Cheers
Brian