Providing we assume the obvious, it's properly plugged-in from the iPad to the controller (no insult here just going back-to-basics), and you have confirmed the USB cable you're using is good, try a different "known to be working wire". I would then uninstall the DJI GO app, make certain you are on the latest iOS, then reinstall the DJI GO app..
Also make certain nothing else is running on the iPad other then the DJI Go app.