It could be the phone or it could be a bad card. The only way to know would be to try a card that is new, never used, or one that you know for certain works in another device and put it in yours. If it can read that one, then your card is the problem. If it can't read any other cards, then the phone is the problem.
Also, please note, if the card you are trying to read is from a device that had the card set as internal storage or encoded, then your device would never be able to read it.