Yes, you need to degauss the monitor probably. I'm assuming when you mean "ruined" is that you get weird colors as you move the door. It is not because the door has magnetic charge, it is actually the other way around. The door's metallic content is causing a disturbance in the magnetic field the monitor creates (sounds like a star wars thing). Degaussing the monitor evens the field with the surrounding magnetic fields by introducing a bias to compensate for it. I bet if you rotate the cabinet around a bit with the door closed, the patterns on the tv will change. It is rare that a newer monitor would not have a degaussing coil around the edge of the screen (usually some sort of metal band going around the monitor). Does your monitor have this coil? If it does not degauss inself at power up, then there's probably a menu function to do it, just look for something that looks like a magnet on one of the menus.