First, thanks for your interest and for registering for a trial.
We've just re-worked our enterprise trial and content community to run on our Share platform. It is actually still in beta and we are updating it frequently. You most likely attempted to log in during a server re-boot. We try to limit re-boots to off hours but sometimes it just can't be helped.
Please try again and let me know directly (nancy.garrity@alfresco.com) if you continue to experience problems. I would also like to hear your feedback on the new site.