If you are trying to save data from it, then I can't help.

If you just want to try and make it useable, and can at some point 'see' it, try this;

Download and install the HP Drive Key Boot Utility - http://h20000.www2.hp.com/bizsupport...X-UNITY-I23839

Open the desktop icon and select the correct flash drive under device.
Select the file system you want to format to (FAT, FAT32, NTFS)
Tick Quick Format.
Click start.

Officially this software (HP Drive Key Boot Utility) is meant for HP disks, but unofficially it seems to work fine at formatting most flash media, even digital camera cards.