When I insert an SD card into the iMac nothing happens. It worked fine until today, I double clicked on the card icon on the desk top and it disappeared! Any suggestions?

When I insert an SD card into the iMac nothing happens. It worked until today, when I double clicked on the SD card icon on the desktop and then it disappeared! Cannot find in "Finder". iPhoto does not open upon inserting card. Reformatted card with device (camera). Used Time machine to restart computer before the double click occurence...did not lhelp. Any suggestions?