Can't logout, nobody can login
 

I'm running 3.7.3

I can't log out, or login to the admin CP [user has admin rights]. If i open a new browser [IE with no cookie stored, I can login as admin, but then it redirects me back to the login page]

no users can login and post, but those who have a cookie can post.

VERY weird.

I was fiddling with the vbulletin options earlier in AdmnCP, but nothing that I can think of that would cause this, and I wish i had a backup of the datastore table [presumably that's where the settings are]

Please help if this sounds familiar :(

Cheers