'''++@jadeandrews++'''
First off you will need to give us a model number for your TV. There are a few things that can cause this. In general it is the mainboard that tells the power board to turn on the backlight for the panel. The main board is also the board where the video and audio are encoded, The signal then goes through the T-con board into the panel. Quick and dirty description :-)
Since you have audio , you know that at least part of your main board still works. Now, in a darkened room turn the TV to a station where you have audio. Shine a flashlight at an angle against the screen. See if you can make things out like silhouettes, menus etc. If so then you know it is your backlight (bad power board, bad backlight array). If you do not see anything then there could be an issue with the main board or the T-con board.
Of course there is always a possibility that this is a bad panel. Anyhow, let us know what you find. Also, if you remove the back from your TV, post some good pictures of the boards and the interconnect cables so that we can see what you see. [guide|21499]