Bob Boudreau , Jul 23, 2010; 06:49 a.m.
Is the lens fully clicked onto the body? It should not rotate if it is. Might be worth a look. When you removed the lens did you touch the metal contacts at the back of it? It doesn't take much dirt/sweat to affect the electrical contact. Perhaps wipe them off with a clean cloth and re-mount the lens. I was shooting this week with a friend who had a Nikon and it wouldn't focus when he changed lenses. A quick wipe of the lens contacts fixed it.