Be sure that the camera is on. Preferences may be set to the incorrect COM port. Change the COM port settings to Auto-Search or to a specific COM port. The Mode dial is not set to Connect. Set the camera's Mode dial to Connect.
Batteries are low or empty.
Replace the batteries. Pictures remain in the camera's memory when the battery is removed. Camera is not turned on. Turn the camera on and verify that the green Ready light turns on. Shutter button does not work.