On the homepage of most of our sites you'll see a Global Tag Cloud. It's a powerful feature that aggregates posts from across all blogs. We've been alerted by a few customers that the global tag cloud has stopped working on their sites, and instead of displaying a page of aggregated posts for a given tag it displays a 404 File Not Found Error.
The solution is simple. In order to display the aggregated tags the system needs a page to display them on. This page is called "Tags" and is occasionally deleted by users who think (rightly) that it is not in their menu structure and therefore (wrongly) is not needed.
To fix the problem, go to Pages > Add New and publish a new blank page titled "Tags".
If the problem persists, raise a support request.