Several days ago I had results on all the keywords I was tracking. They've been slowly vanishing and now I have no results on keywords I used to be in the top 10 results for.
If it's only been since August, and assuming that the pages were new at that time, the dropping rankings is probably due to Google placing new stuff more highly than it eventually merits after one or two updates, which is normal.
That's pretty normal... if you see them drop down in the SERPs instead of disappearing, that's usually worse. Disappearing usually means they were missed in the current index for whatever reason.