A simple error caused by a minimal problem ;-)
The odds are you or someone on your team was playing with master pages and set the minimal.master as the default master page. As the minimal master is not a complete master page, some of the place holders needed by many pages are missing.
To fix, just go to SharePoint Designer and reapply the v4.master (or which ever master you normally use.)
Site Actions, Site Settings
The Settings page has a fall back master page (Simplev4.master) to use whenever there is an error loading a master page. So if you don’t have SharePoint Designer available, and you are using a site based on a publishing template, then you can just navigate to the Site Settings page and pick a master page there.
or just go directly to:
Note, this only works with sites with the publishing features enabled.