Changing site to multisite gives signup page as default

Hi there
I've changed my existing site to multisite tonight, and followed the instructions in the step-by-step guide you have provided very carefully... However I've got a problem which it seems other people have had which now means my site's completely broken - every page is now redirecting to mydomain.com/wp-signup.php?new=mydomain.com
From reading other people's cases it looks like it's a conflict between http://www.mydomain.com and domain.com but none of the solutions those cases mention has fixed things...
I'm really anxious to get my site back up asap - if you can tell me how to do it I'd be v grateful

thanks!