From NHS Evidence: "Please note the Healthcare Databases Advanced Search (HDAS) site will be unavailable from 17:00 today (29 May) until 09.00 on 30 May when the new site goes live. This period of downtime is necessary to upload the new databases onto our systems."
You should still be able to access the baseline resources eg medline and Cinahl via Ebsco etc. You can find a list of all the resources on your My Athens page. Go to the Journals and databases page, under the NHS Athens section sign in with your NHS Athens username and password, and page down to the My Athens Resources section.