My company site has a few high ranking pages for respective keyphrases in Google, Yahoo and MSN for more than 2 years. But 3 of them were gone in Google a few weeks ago. The other pages still rank high in Google and Yahoo and MSN love all our pages as well. We did not do anything on these 3 pages since they came online.

If I search some specific sentence that only exists in one of the pages, Google returns no result. I know page may rank high or low at any time, but it seems that these 3 pages are not in Google's index any more.

We lost some traffic and enquiries from such problem.

How come Google may drop the 3 pages of 2 years old completely ?

Your comment or suggestion is highly appreciated.

Thank you.