Invalid format message when trying to use the RGB(PC) port
Honestly, that sounds like one of two things happened. Either the resolution of your computer video card changed or (I'm not sure with your specific TV) the TV's resolution changed.
If through the TV menus you can adjust PC resolution, I would try changing the resolutions. In the LG 47LB5DF TV, press Menu, then go to Picture, then go down to Screen. On the screen menu, you will see Resolution, Position, Size, Phase, and Reset. First try adjusting the Resolution to some of the options you see. If that doesn't work, try to reset the screen settings.
If adjusting those settings on your TV does not help, then you will have to change the resolution through the computer. Try connecting a monitor to the computer. If you get a picture through the monitor, on the desktop right click and select Properties. If using XP go to the setting tab, if using Vista/7, select Display Settings. Lower the resolution to something you think the TV will be able to handle, then connect the PC to the TV again. If you are using an HDMI connection to the TV, you may have to turn off the computer, connect the cable, then turn the computer back on with the TV on the correct input.
If the monitor doesn't show the picture or if you don't have another monitor, you can try adjusting the picture in safe mode, by pressing F8 after you press the power button, before you see the Windows loading screen. Press if multiple times if necessary. You should see a screen that gives you a few startup options. Select Safe Mode. After Windows boots up, you should still see the screen as the resolution would have been reduced. From there try changing your display settings.
Hopefully that info has helped. I noticed that you asked this question 2yrs ago, so I hope you have already figured out how to fix your problem. I simply wished to post this in case it would help you or anyone else had a similar problem. Good luck and take care.