Someone left the organisation but before leaving, he locked all the files for an unknown reason.
How do you unlock them all so that the other developers can work?>
If the developer has left the organization, then the best thing to do is to delete their workspaces. This will unlock the files for you but also free up some resources on the server.
See the following blog post I did on the topic when it happened to me a few years ago.
http://www.woodwardweb.com/vsts/unlocking_files.html
You can either delete the workspace using the command line (tf.exe) or you can use the excellent TFS Sidekicks from Attrice.