Login has changed inexplicably – popup window says 'WordPress protection'

Previously, clicking the ‘login’ link on my wordpress site has taken me to a separate screen, where I’ve put in username and password and all has gone as expected.

Today, a pop-up dialogue box has appeared, and my username and password (which I’m sure about, and have double checked) doesn’t work. The message received is ‘A user name and password are being requested by http://www.wordandspiritbristol.org.uk. The site says: “WordPress Protection”

I don’t have the WordPress Protection plug in installed.

I’m using Firefox 19.0.2 on windows 7 for a browser., but it doesn’t work in Internet Explorer either. The message is slightly different, referring to http://www.wordandspiritbristol.org.uk at WordPress Protection.

I’m at a bit of a loss to understand what has happened…