If the PC display is not appearing on both the monitor and projector (or on any connected secondary display) it is possible that the device is in Extended mode. To return the device display output to Duplicate mode, hold down the Windows Key and press P. This will bring up a display settings menu, usually with four options.
The four options are:
PC Screen Only - Only shows on the monitor
Duplicate - Shows the same picture on monitor and projector/second display
Extend - Uses the monitor and the projector/second display as two discrete displays
Second Screen Only - Only shows on the projector/second display
You will want to select Duplicate and press Enter (or click on it). This will cause your screen to blink momentarily as it sets up the displays to copy one another.
