Really, there are only 3 things that can go wrong.
1) SD Card needs formatting in the device you are using it in, as already suggested.
2) You must seriously check that you are using the correct SD card for your Device i.e. SD, HD, Super HD & so on. They simply will not behave when trying to cross them.
3) Buy another I'm afraid (as suggested), SD Cards are not the most reliable storage in the world and if it has been used in another device, it may not allow you to use it in any other, I have come across this problem personally.