Nikon d3100 camera just shows a loading screen and won't take pictures?

I have a Nikon d3100 camera and whenever i try to take pictures, it just shows a little hourglass figure (loading screen?) and won't take a picture. It won't even let me see my pictures that are on the camera. What's wrong with it?

A camera will hang up (load forever) if the memory card is full. Try a different memory card.