Hi
What niche was this in? Single or long tail keywords? Any geographical qualifiers at all? What country level Google did it happen in? UK Google results are all over the place at the moment. Lots of movement at the end of last week and it appears to be going through a refresh again today.
Have you checked google webmaster tools for any error message or issues?
Do you know when the ranking drop happened? If it was Tuesday - Thursday last week it is likely to have been caught in the latest update.
More information would help in terms of working out the issues.
