It could be the card and it could be the port. The only way to tell would be to try using the card on a different device to see if it can be read. Then try a different card in the phone.
If the card works in the other device, it is likely the phone. However, if a different card works just fine in the phone, then it is likely the SD card is no good.
Cards can go bad. It happens. If this is a new phone, take it back and have the place you purchased it from either replace the card, or replace the phone (depending on which is the cause). If you are past that point, then contact the manufacturer about the problem if it turns out to be the port and not the card.