In order to fix this, you can try several things. Go to Start and depending on your OS version, you need to run Command Prompt using Administrator rights. It can be found in Programs > Accessories. To run it as Administrator, right click on it and select Run as administrator. After you open it, type
X:\>attrib -r -s -h /s /d *.* where X: is the letter for your pen drive. Now you should be able to access your files.