Endless Admin login redirect loop –

Our site’s been working fine for months and we’ve made no changes to the site recently. But now I can’t log in:

Attempts to access the admin system began failing, returning endless redirect loop messages in all the browsers I try – it seems to loop between wp-admin/ and wp-login.php.

I’ve cleared all the cookies as suggested in some threads but with no success.

The front end of the blog, all the public content, works fine, it’s only the admin side which is effected

The site is not using .htaccess redirects.

Nothing on the server itself has changed.

Any suggestions on where I start looking for a solution? This is a client site, so I need to get this fixed asap.

Thanks.