Unless you have a physical write protect switch on the drive, or you have used some software to password protect the drive, then you probably have a dead drive on your hands.
What does the owner's manual say? Did the drive come with any software? Did you reformat it at one time?
Try repairing the flash drive to see if that works. And as CanadaRAM said, check to see if you have a physical write switch on the flash drive. I turned the switch on my flash drive on and I got the same symptoms as you did.