Does the card work in other devices? Also, you may want to try a different card in the phone and see if the same thing happens. If it does, there may be a problem with the port for it on the device you are trying to use it in.
If the card won't work in other devices, then I would say the card is the problem and needs replacing.
NOTE: It is better to have a card for each device, rather than trying to use on card in multiple devices. Taking them in and out of the different devices can damage the SD card.