As you have probably already guessed, it sounds like there is a problem with your sd card. The fix could be as simple as removing and then reseating it, or the edge connector could be dirty (especially if the person who inserted it touched the gold contacts with their fingers). To eliminate that as a possibility, gently burnish the contacts on the card with a soft pencil eraser and then reinsert. In any event, I would make a full copy of the card onto your computer as a safety backup in case the card itself is going bad. GL!