When a monitor is unplugged (including the adapter as well if applicable - leaving the adapter leaves OS X thinking the monitor is still plugged in) all the Windows should move onto your main screen. Occasionally some windows (eg. Firefox) will keep their position on the very far right of the screen leaving you just enough room to grab the title bar and move it where you'd like to.
One method of arranging the Windows so you can see them (if for some reason they're staying off screen) is to change to the application that owns the window in question, hold the Option key down and choose "Arrange in Front" from the Window menu. It will then arrange all the windows of that application in a cascade from the top left of the screen.
There are several AppleScripts available (eg. this one at Snipplr) that will also grab every window off screen and move it on screen for you.