Photo Booth cannot open because no camera is attached error appears

Hi,
I installed Photo Booth on Mac OS X but I haven’t been able to launch it in an attempt to join my photos into a single image for sharing. When I try to open it I get the error shown below. Any ideas?

Photo Booth cannot open because no camera is attached or the camera is in use by another application.
Make sure your digital video camera is properly attached and turned on. If you are using the camera with another application, close that application before trying to open Photo Booth again.
