First thing to check is to see if the Mac even sees the camera. Plug in the camera then go to the Apple menu and use the "About this Mac" option, go to More Info... then check the USB bus to see what is attached. If the camera does not list there, then the Mac doesn't see the camera at all. In which case you have a problem somewhere else (USB port, cable, or camera).
One check is to turn off the computer and remove the power cord from the wall for a minute, then plug it back in and restart the Mac. This will make sure the USB ports are reset (you HAVE to unplug it from the wall) in case the USB bus has gone wonky on you.
Second check is to plug the camera and cable into ANOTHER computer and see if it is seen there. This will help determine if you have a camera or cable issue. If the USB cable is a standard USB cable, then try a different cable.
Third, check the menus of the camera for how it acts when plugged into USB. Some cameras have options to connect as a mass storage device OR to set up as a picture bridge device or some other configuration which will make the camera NOT talk to the computer like you expect.
Patrick