Your TVs vertical sweep, otherwise wertical scan, output section has got some trouble with it. there are horizontal and vertical sweep sections there for every picture tube based TVs. If horizontal sweep section fails, you will see a vertical line at the center of the screen, and if vertical sweep fails you will see a horizontal bright line across the middle of the screen.
To locate the faulty section and to get it repaired, you have to open your Tv. Open it, after detaching it from wall power socket. Locate the vertical section. It is easy to locate. There will be four wires going to the main board from the deflection yoke, fitted on the neck of the picture tube. Amoung them, two will be thick, usually colured as red and blue, and the other two will be thin. Follow the two thin wires, and you can see it to be plughed into a socket on the main board. Just put the board upside down, and follow the track of that two thin wires through the printed circuit boar. You will reach to the vertical outpuut section. Usually these sections are provided to work with an Integrated Circuit [IC]. screwed onto a metal heatsink.
Before doing any thing, just look onto the solderings of legs of that Ic for any loose contact. As it is very difficult to locate the loosec contacts, just resolder all the pin soldering at that IC by applying a little more solder. and check if the problem is rectified. As these ICs will heat up while working, it is natural to get its leg solderting loose after some time.
If it doesn't help. check the voltage input to the vertical IC said above. If there is voltage, and still it doest help, desolder it out from the board and go for a same type and number. Replacing the Ic will solve your problem.
If you wants any more assistance, feel free to contact me through Fixya. Ok