On a Dell laptop, toggle the laptop's output display by using the function keys: “Fn” key + “F8” key
The “F8” key varies by manufacturer look for “CRT/LCD” or screen icons on the keyboard. Example press “Fn” together with “F8” for a Dell laptop or try restarting the laptop. If still not working there may be a problem with the VGA cable inserted incorrectly, examine the VGA cable connection the keystone shaped VGA connector must match the orientation with that of the laptop's connection port.
If this fails, call the Help Desk to report the issue.
Press the Window key + P
Click on Apple and System Preferences
Click on Display
Check Mirror Displays