Flash drive shows in My Computer but can't be accessed

Flash drive shows in My Computer but can't be accessed

I have a flash drive. When I plug it into my (Dell, Windows XP) computer it appears in My Computer as an empty drive - not a disk with no files on, a drive with no disk in! Double-clicking drive J: gives the message "Please insert a disk into drive J" (I get the same if I type in "J:" to the address bar, too...)