make sure it doesn't have a physical write protection switch and that you have set it to be read only, if it does have a switch just flick it the other way and plug it back in
Hmm, I have 2 flash drives but haven't found any write protection on it, if yours have one then Frosty is correct if not, try uninstalling the driver of your usb ports and reinstall it back so you can try using it again.