So here's the scenario - Ops have changed the TFS build infrastructure underneath you and the build definitions for an "older" project aren't working.
You open the build definition in Visual Explorer, and lo and behold, there's a warning triangle on the Process tab.
Opening that, and you find that the one of the properties (in this case a deployment script) is now empty and showing an error circle.
The problem is that because it's invalid, the editor has cleared the property - even opening the dialog doesn't help - it's all gone.
So how do you find out what the property WAS so you can fix it?
Well, first, close the build definition WITHOUT SAVING IT!
Next, fire up a Visual Studio Command Prompt and CD to the folder that's mapped to the root of the source code in TFS.
What you need is command line tfpt.exe tool from TF Power Tools (you had that installed already, didn't you?). This has a handy BuildDefinition /dump option that will show you what's in the build definition - regardless that it's invalid.
You can now open the text file in notepad and see what the property WAS - job done.