As someone above said, press Ctrl+F5 (force a refresh without accessing your local cache) and all should be OK. If not, go into Tools/Internet Options and delete your temporary internet files and cookies. Then restart your browser, log in to pprune.org and you should see your normal layout.
Tried all that about 20 minutes ago, no difference, then just now changing to a different Topic, it came back to normal. I would suspect my computer, but PPrune is the only forum site that it happens on. Very strange.