Can no longer duplicate display to external monitor on Windows 7
You should have an icon in the lower right of your screen (taskbar) related to your graphics in your computer (Nvidia, Intel, etc.). Right-clicking that icon you should be able to get to something like "display settings", where you should find an option for cloning your display, or dual-display, or something like that. Using that option should restore the dual-monitor display function.
If that doesn't take care of it, it would be much more helpful to have your laptop's specs and configuration. Graphics card, info, etc.
I had the same issue on Vista. I have a 50" HDTV at home that I watch Netflix from my laptop. Every few days I would take the laptop to work and when I would get home connect the HDMI cable and the screen was cloned onto the 50" HDTV.
In my taskbar I found "Intel(R) Graphics Media Accelerator Driver for Mobile".
- then I right clicked choose "Graphic Options"
- moused over "Output to"
- moused over "Clone Displays"
- clicked on "Built-in Display + Digital Television" and resolved my issue.