How do I fix a write protect error to a pen drive?

My computer is an Acer E500 series, 250Gb HDD, 1Gb ddr2 667 ram, LG DVD Ramdrive, the USB port used for the pen drive was 'L'. The name of the pen drive is a Shimano with 1Gb, I think, I am sure thought, the pen drive has no protect switch on it.
My friend was using my computer to update and rewrite a document in Microsoft Office Write, from her pen drive and when she tried to save the document back to the pen drive it came back with this write protect error that has prevented her from taking the file with her on that drive. I still have the file on my computer and in addition I've done a complete backup of her pen drive to my HDD, during the course of which, we received a corrupt file error that turned out to be a temp file that was saved to the pen drive during the creation of these documents, could this be the cause of the problem or is there something else I'm not aware of that's happened? Thank You, John A.