Turn the phone off.
Remove the battery
Remove the SD Card
Use a CLEAN pencil eraser and LIGHTLY rub the contacts on the memory card and the contacts inside the memory card slot in the phone.
Replace card and battery, turn the phone on and that "SHOULD" correct the problem. Make sure you are putting the card into the phone correctly (some cards go INTO the hinged door, not UNDER it), so make sure you are inserting the card correctly so that it has constant contact with the contacts inside of the phone, and does not move around and lose contact if the phone is dropped or jarred.
If this does not work there are more steps that can be taken to help, but we can get into those in email, text or voice, if this does not work.