buckworks - 6:40 pm on Jul 19, 2004 (gmt 0)

Don't be too quick to panic. I had something similar happen a few weeks ago, but it had a happy ending. Over a period of a few days, hundreds of pages simply vanished from Google's index. I was busy with other things and didn't have time to try any tweaking. Two or three weeks later, with no action on my part, the pages began appearing in the SERPs again, and since then things have seemed normal.

What's happening to you might just be one of those nobody-knows-why fluctuations. Keep about your normal work and wait a few weeks before you try anything drastic.