It may be that when you unplugged the DVD player, it lost the output settings. You might try going into the DVD's Setup Menu and optimizing the output to at least 480p, and enable the progressive scan feature. If you can get a higher resolution, that's even better, but I'm guessing you have a progressive scan DVD rather than an upconverting DVD player.
I have a PS50Q91H - the 50" Plasma unit from the UK / ME - we had bought this overseas and brought this over - works fine here with a Marantz DVD player that I connected using an S-Video cable on the side panel of the TV.
A few days ago I connected up my laptop to the TV using an S-Video cable - connected without any issues using Fn-F5 on the laptop.
Two problems now:
1. When I connected the DVD player back player to the TV, the picture is blurry - nowhere near the sharp quality we used to see before. The picture looks like a regular old-world CRT TV.
2. I am now back to trying to connect up using a regular non-S-Video cable - the pictures are sharp, but it seems to me that the it sort of oscillates between very sharp to soft - this does not make it unwatchable, but I am able to notice it.