Hi!
So, it does sound like the Wii U gamepad is no longer synced. It does sound like software related instead hardware related.
When you press the "Sync" button on the Wii U it self, what does it say? Be aware that you need to press it once more again (on the Wii U it self) when syncing the Wii U gamepad.
One press on the "Sync" button: You can now register a Wii Remote.
Two presses on the "Sync" button: You can now register your Wii U gamepad.
Press the sync button on the Wii U gamepad, and then the gamepad should ask you for the symbols to input.
On the Screen it self, it should show you the symbols, not on the gamepad...
Another workaround is to sync a Wii Remote or Wii U pro controller. Then press in the right upper corner to switch the displays. This can help you to register the gamepad... Usually you can switch with the "X" button, but as you cannot use the Wii U gamepad, this will be impossible to do (except if you have the Wii U pro controller).
If you can share a video with us, that would be great!
Please let us know how it goes and if you have any questions!