First of all, is the camera on and set to playback mode? Sorry if that seems obvious but it has caught out others in the past.
If the camera is on and set to play and then it could be a cable issue.
If you open Image Capture with the camera connected to the Mac via USB cable, does this application see the images on the camera?
If Image Capture also fails to connect to the camera then it could be the USB cable at fault, so try another.
If you have tried a number of cables already, open System Information, select USB in the left column. The top right pane will show all the USB busses on the Mac. If the cable is okay, then the camera should be listed under one of the USB busses. So do you see it?