1. Open Start Menu >> Run, type regedit and press Enter, this will open the registry editor.
2. Navigate to the following path:
3. Double click the key WriteProtect in the right pane and set the value to 0 in the Value Data Box and press OK button
This will change the value in the registry and let you use it again.
If this is not fixed after the registry mod run a checkdsk from dos to the pen drive .CHKDSK X: /F (x is you pendrive letter)
Last solution , is to format the pen drive DO NOT do that if dont want to loose your data.
Hope this helps.