Try to shake it awake by shift-refreshing the homepage to get a fresh cookie. Some folks report that they can't get in until they clear the old ones and empty the cache: in Firefox and IE, head to the Tools menu and the delete/clear history option. In Safari, it's under the Safari menu, as "Reset Safari."
(We're still working on some of the tail-end issues, so don't panic if you still can't get in! Lots of stuff in old posts will be broken–links to profiles, for example–until we give the all-clear on the upgrades and hit "epic republish" on many years of posts.)
