Laptop not working with second monitor

My Inspiron 9300 no longer works with second monitor, a Dell Flat Panel display. The two screens no longer work simultaneously.The second monitor worked perfectly with the laptop for several months. The settings in display properties seem to be right. Could I be missing something?

Both monitors have screen saver and desktop background. Both monitors work with mouse, but task bar, icons, and programs show only on the laptop screen. Individual icons, etc. CAN be dragged over to the second monitor with the mouse, as can programs or parts of programs. But then they are gone from the Inspiron screen.
Thank you for any thoughts ...