If its the one which you got from nokia, forget it. Nokia cards hardly lasted for around 1.5yrs. :ashamed:
Best solution is to connect it to your pc via card reader and format it. Connect it from your backpanel usb.
After format, check its integrity or do checkdisk or a thorough scan disk via windows.
If that doesnt work, try another cellphone to get it formatted, it will surely work. On my phone i have successfully formatted such corrupted mmcs from nokia 5800, 6500, n72, n97, n73, 5530 and few others.
But even after that there isnt a guaranty that these cards will further work for too long. Coz after that, friends got issues like corrupted videos/images captured & choppy mp3 listening. In between cell gets very slow. :|
AFAIk, sadly, this issue only arises with nokia.
This is a symptom of an dying mmc.
So try the above methods ftw.