Are you toggling the projection mode?
- Press the "Win Key + P" to bring up the Projection Mode menu.
- Or type "Connect to a Projector" in the Start Menu Search for the Projection Mode menu.
You should have the option to Project or Duplicate desktop.
The Acer Aspire One that I am familiar with will not Span Desktop.
And if the projection mode is set to "Computer Only", there will not be a signal on the VGA port.