There are four common solutions to this problem.
1) Choose the “init SD card” or “change SD card” functions on your printer to refresh the system.
2) Try using a new SD card or a wiped SD card.
3) Take a rubber eraser (found on the back of a pencil) and rub the metal contacts in the SD card reader.
4) It is possible if nothing on the list has fixed the issue then the SD card slot is broken and will need to be replaced.