The first things I would try is both of the following in no particular order:
Viewing the card on a PC using a card reader. If you can view the card, backup the files to the PC and format the card as Fat32. Then put it back into your device and see if it's recognized.
Try the card in another device. Have a friend or family member let you put the card in their device. If the card isn't recognized in their device either and formatting the card doesn't help, the card is probably bad and needs replaced.
If the card is recognized by other devices, your phone probably has a hardware issue and cannot read the card. In this case, take your device into your carrier for repair or replacement.