Yesterday, a few advertisers reported at WebmasterWorld that they were unable to see data at the ad group level, after logging into their Google AdWords accounts. They were able to see data at the campaign level, but not at the Ad Group level.
After a couple of hours or so, the problem seemed to fix itself. Well, someone had to call Google for support and Google told them to clear their cache.
So if you have any issues with this problem, first try clearing your cache and/or restarting. That should clear things up for your day.
Forum discussion at WebmasterWorld.