There’s an attack on WordPress websites currently unfolding. A large number of websites have been significantly compromised. This type of thing is rare, but it can happen.
If your site has been hacked, only the front page will be functional, you’ll see strange code at the end of your permalinks when you hover over links, and on the Users page in the dashboard the number of administrators will be given as 2 even though only one will be listed (unless you’ve created some more).
The vulnerability used by this attack isn’t present the latest version of WordPress, v2.8.4, but it is present in very recent releases. All WordPress users who haven’t already upgraded to v2.8.4 should therefore do so immediately.