Wii U locked

I bought a second-hand Wii U, and when I went to the Network ID, I had to choose the user and enter the network password or the secret code, but I don’t know them. If I want to receive the code by email, I can't since it's not my email address, and I can't change it. I also can't return to the home menu, so I'm totally stuck. We've already tried turning it off, unplugging all the wires, and when we turn it back on, it always goes back to that. Help me please!