Unfortunately, but there's likely nothing you can do to fix this and that your GamePad and/or Wii U console may need some sort of repair, sorry.
So probably the best thing to do is give a call to Nintendo's 1-800-255-3700 (6 am ~ 7 pm Pacific, everyday) Customer Service number, or use their webform to contact them, and one of the service representatives can discuss repair options with you.