I have an lg 42inch plasma and now a vertical black line running down centre of screen about 2 inch thick

I had the same problem with a five year old 50" LG Plasma. I fixed the problem with a 11 dollar soldering iron from Radio Shack and about 2 hours of my night. You have to open the back of the tv and locate where the ribbon strips are attached to the screen. I beleive that on your 42" they will be on the bottom of the tv. Mine had 11 on top and 11 on bottom. Yours will probably have around 7 or 8 along the bottom. Mine was a little easier since I did not have to remove the control board. You unfortunately probably will have to remove it. It is not hard at all, just takes more time. Make sure to keep your screws seperated and remember where everything goes. On bigger jobs some tape and a marker are very useful. Put tape on the board and around the wires removed and leave a tail. This way tou will remember to hook them all back up. Once you find the ribbon strips you have to remove them one by one and look at what is behind the metal place. There will be 4 capacitors and 4 resistors. The tv manufacturers want you to buy new tv's every 5 to 10 years so they use a very cheap solder on their contacts. Use you soldering iron and re-heat the solder to liquify it again and thats it. Allow time to cool on each of the caps and resistors. It took me about 1 to 2 seconds on each side of these parts. I did use a dual wattage soldering iron from radio shack. Its is a 25 and 35 watt iron. I used the higher setting. I have pictures if you would like me to email them to you and show you what I did. Be careful removing the ribbon strips, I broke one of the clips that hold in the ribbon strip. That is costing me an extra 25 dollars off ebay but still for a free tv, its not bad.