This is SO stupid. I did the same damn thing trying to setup a dual monitor display on my laptop. The external monitor wasn't displaying (first time doing this) so I unchecked the laptop one to try and force it over..... yeah didn't work. The ONLY way now the laptop screen even comes back on (luckily) is if the external monitor is not plugged in!
There is NO way to get the option back of the two monitors EVER. I removed ALL nvidia software, from EVERYWHERE on the machine, even hidden files and in the registry then did a CLEAN install option from Nvidia and STILL does not work.
This is poor form on Nvidia's part not to have a reset to defaults option in the multiple display driver area to rectify this and to have it so hardcore encoded that even a driver reinstall doesn't fix it.
Don't tell me I have to reinstall windows just to fix this? If so this is the last Nvidia card I will buy, people go on saying Nvidia drivers are awesome, BS.