The earphones will turn themselves on automatically when removed from the charging case.
If they do not, please ensure your case is charged then put them back in and try again.
To find out how to charge the case and earphones please see this FAQ.
If the Melomania 1+ are in your ears and they powered off, hold down the button on each earbud for 2 seconds. You will then hear the voice prompt say: “Power On”.