Site locked out

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Hi all,

I hope someone can help me with a serious issue that's just arisen.

I was working with the advanced permissions for a site I'm developing and now if I visit it I only get the Sign In screen, the site itself isn't displayed.
If I sign in as the site admin I can see all the pages etc.

Does anyone know what I've done to cause the site not to display, and therefore how to cure things.

--EDIT--

A little more information.
I have another user set-up on the site with administrative access.
It was due to this user that I was working with advanced permissions; limiting what they can do in the site as regards adding blocks to specific areas and limiting editing on specific pages.
If I log in as this user I get the Dashboard, but if I click on the Sitemap there is nothing there - it's completely blank. If I then click on the Return to Site button I am returned to the Sign In screen.
This is very confusing...

--END EDIT--

Thanks in advance for any help.
Tony.

--LATEST--
Ended up setting up a whole new site and basically recreated the site form scratch. Time consuming but at least it works now!