can't access Admin after moving to a new web host

We had a developer build a site for us, it worked on their server, they zipped it up and I put it on our host.

It is running just fine but we can't log into Admin.

I can log in as a user and edit existing pages but we get "Access denied" when attempting to access any of the Admin areas.

The developer -- who is a bit short on time -- looked at it and said "it all looked fine" and had no solution.

Any ideas where we should start looking?