Either the flash drive is broken or your reader is broken.
A little more detail would be helpful. Is a "flash drive" an SSD, a USB thumbdrive, an SD storage card, or what? Did it used to work? Does it work in your camera or whatever? What kind of device are you using to read it?
With what little you have provided, the situation where I see this is usually when a flash-card reader is plugged into the PC but no flash card is in it. But I doubt that that's your issue; you wouldn't need help to figure out that you have to put the card in!