Why won't Windows 7 detect and use my second monitor?

My main computer set up is a dual-boot, dual-monitor rig running on a AMD Radaeon 6800 graphics card. This system was working perfectly until today. Now, Under Linux and during Windows boot, both monitors are active. Linux extends it’s desktop to the right hand monitor and the left monitor is useable just fine.

Under Windows now, the primary monitor is turned off and will not come on. The secondary monitor is acting like a primary and when I look into any of the Windows settings, including the Catalyst control center, the secondary monitor is the only one detected.

I have made no settings or configuration changes, and it works just fine under Linux.