Complete format it by plugging into a card reader > My Computer > right click on the SD card > format > uncheck quick format
You should always have the camera format the card.
a) Copy off the video if you wish to keep it, then
b) FORMAT the card by the camera (if not done already)
Also, some cameras can't make use of more than 2GB so you'd need to check your camera manual to see if you can fully use the card.