Hang on to your hat.
OK, if this was mine I'd boot safe mode then head to control panel, device manager (this varies with window versions but you know where we are going so you have to get there.)
Now find displays and remove them. If there is an uninstall do that too.
Light reading:
http://www.bleedinedge.com/forum/showthread.php?23682-How-to-REALLY-show-hidden-devices-in-Device-Manager
https://support.microsoft.com/en-us/kb/315539 is for XP, but there are similar for other versions.
Finally I reboot and hopefully Windows on it's redection will get the parameters of the display right. If not I'm off to the display's web page to look for a monitor.inf or driver.
We had our computer hooked up to our nice 20" ViewSonic VT2042 1600x900 resolution monitor. Then unfortunately we had to switch our ViewSonic out with another cheap, crappy yet slightly bigger 21" Toshiba 24L4200Ua monitor because someone else needed the ViewSonic so it could fit where the Toshiba wouldn't. We eventually got our ViewSonic back but to our dismay when we hooked it back up the computer, it now still thinks that it's on a 21" screen for some reason and won't go back to 20". We have tried changing the resolution every which way and we can't get it to go back to 20"! So frustrating. Anyone have any suggestions?

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