Did you attempt to mod your console? If so, the issue could be that it's bricked. ( Although to my knowledge, bricking usually results in the console not powering on or properly reaching the start screen. ) If you know anyone who has a Wii-U and has resolved the issue themselves, or simply an electronics technician ( such as Geek Squad ) then your best bet is to ask them.
Alternatively, you can try taking the MH3U disc out of your console and then trying to change settings. If you still freeze, refer back to the first point-in-case. If you really want to try and make a quick fix out of the things though, just take the Wii-U back and get it exchanged for a new one under the claim "faulty system" or "malfunctioning system". You should also try checking to see if all your cords are plugged in correctly, namely the visual cables. Besides those, see if all the power conductors are connected properly, and make sure that your TV is properly displaying other high-stress visuals ( super-CGI movies like Avatar or simply HD movies, Blu-rays or whatever.)