I dont think they have a specific contact support for your issue.
Did this just happen recently after their index update?
Take a read of
Matt Cutts post on their update and then the
Sitemaps post which gives "some" insight on what could cause the de-index.
Just out of interest do you have a Google Sitemap for the de-indexed site?
You could try and ask Matt Cutts via his blog but its a long shot.
If you want post the url here and see if anyone can spot the cause.