I can't give you details because you didn't say what version of Windows you have, but you should be able to use System Restore to go back a day or so prior to the problem.
I have Dell latitude laptop and a Samsung flat screen tv. Hooked it up and it worked great for one show, Star trek voyager off the net. Went to another episode, but it said the settings were wrong. Went to display in control panel and fiddled around. Now I get the wallpaper, but not the desktop. I click on extend to second monitor, but the apply button is dark. I click ok and nothing happens and when I go back to display/settings, it's back to first monitor. How do I get this working again?