My Aperture 3 library has vanished
PS I am running on a Macbook air with OS X Lion 10.7.3.
are you sure you are viewing the right Aperture Library? The name of the library you are viewing is displayed in the upper left corner of the window.
If it is not the correct library, go to the Aperture menu bar, press "File -> Switch to Library" and look at the drop down menu, if you see your correct library and switch to that.
If you are viewing the correct library, then maybe your images are hidden: Look at the search fields in the Browser and the Library Inspector, if any search rules are set; if yes, clear the search fields.