There was a problem with some localised builds of WordPress 3.9 – including UK English en_GB, where it switched the site over to beta testers mode and upgraded to an Alpha build of 4.0.
For me it didn’t look different, but for some people it broke plugins.
Here’s how to get back to normal
1) Using your FTP client – download version.php from the wp-includes folder.
3) Upload it back up and browse to the updates page
As of 9th May 2014 9:24 (and probably a few hours before) it will tell you 3.9.1 is available in your language. Update and you will be back to normal.
The WordPress peeps have fixed the bug in some of the localised versions.
If that’s beyond you – get in touch I can help!