This appears to be some kind of water damage to the logic board(s) since everything is not registering input. Best thing to start with is to open the device and see if there is any kind of residue or dried water damage on any of the boards. I would suggest following this guid: [guide|57152] to get at all of boards in the device to inspect them.
From there, the next best solution would be buying a busted pad off eBay and swapping out the boards or seeing if there are replacement boards for a reasonable price.
If any of this information helped, please rate it and good luck troubleshooting your Wii U pad.