PubMed transitioned completely to the redesigned interface effective today. The NLM Technical Bulletin article has more details.
To keep up-to-date with major changes like this, sign up for automatic email messages by becoming a subscribed member of PUBMED-ALERTS, an announcement service available from the U.S. National Library of Medicine (NLM). To subscribe, send e-mail to firstname.lastname@example.org with “subscribe PUBMED-ALERTS [your full name]” in the body of the message.
And, if you subscribed to PUBMED-ALERTS, you would get the message later during the day that said: “We are having technical problems with PubMed. We have reverted to the old interface. We expect to transition to the redesigned version in the next few days.”