The details shared here apply to the most recent versions of our themes. To keep your theme up to date, check out this helpful document.
All Out of the Sandbox themes include an option to enable an 'Announcement Bar' at the top of your store within the Header which can be setup in Customize > Sections > Header (or Customize > Sections > Announcements in the Flex theme).
The announcement bar includes an option to close it by clicking the 'X' in the right-hand corner.
If you've closed the announcement bar and want it to reappear, you will simply just need to clear your cache or go into 'incognito mode' in your browser. Otherwise, it will reappear on it's own in 30 days.