Did your fellow broprogrammer leave for the day? Did you reformat your virtual machine or development workstation? When a file gets checked out in TFS, it gets associated not only to a user, but to his/her workspace (the machine/source code folder they are on). That makes it slightly more difficult to regain access to the file and undo their checkout so that you can take over.
This task is not uncommon and has been blogged about. Huge thanks to Richard Murillo for documenting it in an MSDN blog here: http://blogs.msdn.com/b/rimuri/archive/2006/03/06/544686.aspx
The date of that post is 2006, so here we are 8 years later. Allow me to regurgitate it for you, with full credit to Richard, just in case that post gets removed.