Is there something I can do to repair my Symphonic TV which has sound but no picture just a light line in the center of the screen?

Time to buy a new one... there is nothing you can do. In fact its extremely dangerous to play with the insides of the TV. If you have to ask how to fix it... you don't have the skills, equipment or tools to do it.

Before buying a new one, make sure it's on other inputs too, and test with a known working source, such as a DVD player that works fine on another TV.

Does it appear over the main menu or under it? As smoothy mentioned, it will be dangerous to attempt the repair yourself. Chances are, if this happens on all inputs, you'll be looking at a low end charge of around $500 to have it fixed.