If you go to Start / Control Panel / Display / Settings tab, do you see 2 monitors listed in there?

Can you click on each monitor (click on the large blue boxes labeled "1", and "2"), and see if any of them does not have the "Extend my Windows desktop onto this monitor" setting checked on it?

You might try making sure each of those is checked, and also try flipping which one is set up as the primary monitor.

Also, can you please try rebooting your machine and see if that has any effect?

- Michael