Stylesheet failure & error when editing pages

Happy user of WordPress, generally we get along just fine. However, a couple of days ago things started behaving oddly…

• When I update a post, although the update is successful, I get taken to my 404 page
• The site renders really badly… about two-thirds of the time it looks like there’s just no stylesheet. Repeated refreshing is needed to get a page to render properly
• The “add media” button above the post edit box have disappeared
• Sociable’s links either appear as pizza-box broken-image icons, or as text only
• Images sometimes appear, sometimes don’t.
• (using Sociable) data fed through to Facebook contains the link only, no article excerpt, no image

Something has clearly gone very badly wrong. And I’ve got a feeling it’s something simple, but can’t for the life of me figure it out. Very little has changed, the problems seemed to start when adding the latest post. There were a couple of plugin updates at about the same time, but I’ve tried deactivating ALL plugins and it makes no difference.

I’m running on a shared Linux host, in the CPanel/Fantastico environment, so I don’t have root or command-line access.

What to look for next?