Lets backup a bit here; so your computer does bootup but the main screen is not showing, just the extended desktop?
If that is the case then Right click the desktop on the screen that is visible choose Screen Resolution. Press Detect. This will tell you what monitor you are looking at (eg) 1 or 2. Then choose Multiple Displays and take the drop down menu and choose the monitor number that is visible. You should see your normal desktop on the visible monitor now
Now, go to Start/Search and type devmgmt.msc, right click the devmgmt results and Run As Administrator. In the Elevated Device Manager, expand Display, right click your display adapter, choose Properties/Driver/Roll Back Driver. Choose the last driver that was working for two monitors. after that, Go back to Screen Resolution and Extend the Displays again. You can also do this in your video cards control panel by double clicking this icon in the Taskbar by the clock or the Control Panel.