Eclipse: Missing "Run Configurations" button? (Command Groups Availability not helpful)

Question

I have a weird problem. I'm using Eclipse to make something in Python with PyDev. Fine. But one day, the Run Configurations button decides to vanish; now I can't run my program from within Eclipse! Going to Window -> Customize Perspective -> Command Groups Availability doesn't help, because the Launch Command Group is missing for some reason! I have no idea what could have caused this. I'm using Ubuntu 12.04.

Answer 1

Being the genius I am, I never thought to simply delete my Eclipse configurations. For Linux users, that's in /<your workspace>/.metadata.