I have a Lenovo laptop whose backlight went out. So I plugged in a separate monitor and everything was working well. But the display was giving me a split monitor and I was losing stuff to the margins because it thought I was using two monitors when I was only using the one I plugged in. In trying to get the margins to be readable, I inadvertently chose the dead Lenovo monitor in the Screen Resolution choices of Monitor 1 and 2. Now I can't see anything and I don't know how to get it back to the plug in.
I changed my Monitor to display a dead monitor. Now I can't see anything to revert it back?
Ganip
Unplug the monitor and start then plug it in you can use the blue Fn buttons to change the display
Guest
It's Windows 7. Thanks. Won't the blue button destroy everything? I've done that once before and had to reinstall absolutely everything from scratch and it destroyed all my files. Thanks again. I've also already tried to unplug and replug it in.
Ganip
Function and f7 buttons on my laptop switch between monitor outputs this would be the first thing I would try. If it is windows 7 or older you could try hitting f8 while booting and boot into safe mode which resets all graphic settings to factory minimum. It could have a symbol on it like any in the picture and which f key it is depends on brand. 
nick