Try the following:
1) Make sure you have a component (NOT RGB) cable. If it is coming from the VGA port, and not a dongle, it may be an RGB cable. Note: many component cables have Red, Green, and Blue colored plugs, so make sure. Component video is YUV, not RGB.
2) I would make sure your output in your video card is set to 480i. This is the resolution for standard (non-HDTVs). 480p and 720p are the resolution for Enhanced and High Def TVs. 1080i is for High Def TVs only. To enable this, you may need to set your video resolution to 640x480 or lower.
3) The other thing I would check is possibly you have swapped a connection on the TV. If you plug the wrong plug in, or if one is loose the picture will not be reconizeable.
If none of these solve the problem, you may want to try connecting something else to the component input (such as a DVD Player) that you know is good, and see if it works. If it does, you know the problem is the computer or cabling. If it does not work either, there is a problem with your TV itself.