the problem maybe at your Memory card or your phone. Try to insert your Memory card into another phone. If that phone can read it then there should be no problem at your Memory card.
Otherwise, try to insert a readable Memory card to your phone. If your phone can't read it, then you have an issue about your phone's card reader.
If the problem is in your Memory card, go buy a new one. If the problem at your phone, go to some repaire centre