So I came on looking for a thread I'm sure existed about the gamepad freezing. Well it happened to me today! In ZombiU just before I went to save it, it stuck and none of the buttons would work (except the TV one for some reason), not even the touchpad. At first I was really frustrated and didn't know what to do so I tried loads of different things to get it working again. Eventually, I did! How?
If your gamepad ever freezes, simply take it 'out of range' of the WiiU. It will cause the gamepad to disconnect from the WiiU and tell you that you need to take it back within range of the WiiU. Once you do and it reconnects, it should reset itself and you can continue as normal, without having to worry about losing your progress!