<p>Someone left the organisation but before leaving, he locked all the files for an unknown reason. </p>

<p>How do you unlock them all so that the other developers can work?</p>

For the following operation, you will need to be either a project administrator for the project you want to undo the check-in on or a Team Foundation Administrator if you want to do this across all projects.
If you still have the username of the person, you can simply do something like this:
Open up Visual Studio command prompt (Start -> Programs -> Microsoft Visual Studio 200X -> Visual Studio Tools -> Visual Studio 200X Command Prompt)
Run the following command:
tf lock /lock:none
  /workspace:WorkspaceName;USERNAME
  /recursive $/
To get the list of workspaces for a user, just run the following command from the same prompt:
tf workspaces /owner:username
For more commands, check
tf /?