Works only on one subdomain

I have been running my blog at http://www.matt-fletcher.co.uk/blog/ for about a year now without problems. But today I came to view the blog, and it says “No results found”. I logged in, and there are also no posts shown in the site admin.

I then happened to visit http://matt-fletcher.co.uk/blog/ and found that all the posts were still there! The “www” is just set up as a CNAME to the domain without the www, so I cannot imagine why WordPress would respond differently. In the options, the wordpress address and the blog address are both set to http://www.matt-fletcher.co.uk/blog

Any ideas? This has really stumped me.