Log in from Account using the same login method and email address you used before. This will enable your previous data to be restored and transferred over.
*All gems and medals earned will become invalied after 180 days from the date of your last earned gems or medals.
If your data is not restored even after re-logging in from Account, it is because you logged in with a different login method or email address than you used before. This means that you are now logged in with a different account.
Go to [Account] -> [Account Settings] and select [Delete Account]. After this, try logging in again with your login method and email address which you remember using originally.