What does it mean when the index page is the only page indexed?

Site shows Title & Description, and PR4 but is gone!

[Baraccus]

I have a PR4 site that has been ranked on the first page of the SERPS for my keyword phrase forever. Suddenly, it's totally gone from the SERPS, and only the index page shows up when I search by domain.

The Title and description are still correct, and I'm still showing PR4, but no SERPS, and only the index page seems to be indexed now. Didn't do any updates on it for a long time now.

What does this mean?

Thanks for the help!