There is a small issue in Drupal that can drive you crazy. It's an issue with access denied on all drupal pages. Yeah, I've expirienced this problem lately, but found an answer how to manage this. It's difficult to find out what the problem causes it, but in my case this was exeeded quota of mysql db.
Anyway, the solution. Go to your db and check if the node_access table contains any data. If not, you're almost done. Just run this query and the site should start up.
INSERT INTO node_access
(nid ,gid ,realm ,grant_view, grant_update , grant_delete)