Connect the iPad to your computer with the cable. If iTunes does not start up on its own, then launch it yourself. Click on the iPad name in the upper left corner of iTunes (if you are using iTunes12). You should see a black and white icon that looks like an iPhone/iPad/iPod Touch. Click on that and then you will see your iPad. The Summary Screen should appear in the window on the right. Click on the Restore button on the right side of the window and let iTunes restore the device.
IF that does not work, you may need to use recovery mode. That is explained here.
If you can't update or restore your iPhone, iPad, or iPod touch
If the iPad is not being recognized in iTunes, look for help here.
iPhone, iPad, or iPod touch not recognized in iTunes for Windows