When you reconnect an external monitor used for extended display on a Windows 7 laptop after laptop display has automatically set itself as the only one display, you need to re-enable extended display.
1. Set the Close Lid setting to “Do Nothing” under Control Panel -> Hardware and Sound -> Power Options -> Choose what Closing the lid does
2. Connect an external monitor to a Windows 7 laptop (or dock the laptop to a docking station). This will result in a Cloned display configuration with the laptop display and the external monitor.
3. Use Windows Key+P to extend the display to the external monitor.
4. Disconnect the external monitor and use the laptop without the external monitor.
5. Close the laptop lid, and connect the external monitor (dock) without opening the laptop lid.
When you open the lid, the display only appears on the laptop screen instead of appearing in an extended mode and both the laptop screen and external monitor.
Windows 7 will apply the last known display topology, which is the state of the laptop after step 4 above.
Use Windows Key + P to extend the display again to the external monitor.