Deserialization and cannot checkout files in TFS

Why?
So I get back from annual leave and I can't check-out / check-in any files in a Visual Studio 2010 project. All my colleagues can.

What?
Rather silly I know and the solution is one of those obvious ones for those that know how but for those who can't remember, then it can take a while.

How?

1. Open the solution file
2. Select any file in the Solution Explorer
3. Go to File » Source Control » Go Online

The "Go Online" is the option I was missing in this case.