|First make sure the DVD player-to-TV connections are tight and the cables in good condition.|
If that's not it, I don't think 'magnetizing' is the problem, or at least it's not something that can be removed. As I understand it, the laser is focused using a couple of small permanent magnets and coils somewhat analagous to the coil and magnet configuration found in audio speakers.
However, that setup in a DVD player is much more sensitive and I think what might have happened is the external magnetic field caused a slight but permanent misalignment in that focusing assembly. But I don't think it's because some piece of that assembly became magnetized and thus can be demagnetized. Realigning it may be possible but it'd need to be done in a shop and would probably cost more than the DVD player is worth.