Other than the key combination that Ute posted which shifts the laptop's
output between built in screen and the VGA output, there is another key that
will cause all presentation screens to blank and that is the B key (for
blacking the screen) on the keyboard. If Ute's suggestion doesn't work, try
pressing the B key once.
If these two suggestions don't work, tell us what and how you are
presenting, the type of equipment (computer and projector system) and
version of PPT you are using for further suggestions.