Do the following steps:
1. Right click on my computer icon and open manage.
2. Then open disk management.
3. There you see your pen drive.
4. Right click on your pen drive and click on format.
5. After successful formating restart your computer.
6. your pen drive will work again.
If the above procedure won't work for ya than try the following :
This is a link to the HP formatting tool - it might format your pendrive
HP USB Disk Storage Format Toolhttp://www.pcworld.com/downloads/file/fid,64963-order,1-page,1-c,peripherals/description.html