Most USB drives have a small write-protect switch/slider which you can use to make the drive read-only. If your drive doesn't have one and its essential for you, then consider buying one that does...
There are several possibilities: Check if there is a physical write protect switch on the side of the flash drive. If there is set it to the opposite position it is in now. Your operating system may...
You can usually find a write-lock switch located on the drive itself. If there is no such switch, you can check under "Properties" when the drive is open; if you do not have write access, you may be...