What troubleshooting have you done already? did you re format it using the cars interface? Take it out of the car and connect it to your computer to reformat it? what file system is it currently formatted in?
(pet peeve time.... I support users at my job, and its frustrating when they say "XXX doesnt work", because that tells you almost nothing).
How about some more information so that those who want to try to help you can do so?