Missing Files.

We run our entire company on Dropbox (aprox 20GB). Over recent months we have experienced files disappearing. They are not deleted because they do not show up under deleted files.
Luckily in this instance we have had a hard back-up but this is not the point. The reason we chose Dropbox was to remove the cost and management time of running a back-up internal server etc.

Anybody else experienced this and who can we contact to check for our missing files?