was it visible before? have you previously formatted the drive? what version of OS do you use?
if you use windows 7, go to the computer->manage drives (search in the start bar if you need) Should give you a list of all your drives, this section will let you format your drives and reassign letters if you wish.
If your PC still does not show you your external drive (if that area does not list your drive), get latest copy of Ubuntu and run the disk as a live disk,,, if even the linux kernal can't see your drive, it needs to be replaced --- if linux sees it, reformat drive to NFTS (pretty much anything else won't play well with windows)
yes it was visible and i havent formatted the drive,im using windows vista and i went to device manager under disk drives it shows usb drive instead of the name of the drive(LG) but my colleagues suspect a virus or something do u know what might be the problem?