ITunes Error Message

I somehow lost the library for ITunes. I'm not sure what happened. I receive this message when I try to log into ITunes and when I attempt to make a purchase.

"The folder, "Previous ITunes Libraries" is on a locked disk or you do not have write permission for this folder."

I checked the folder and I can't unlock it. I restarted and turned off the PC without sucess. How do I fix this?