2 connection types were available before July 1st, 2020: 
- Facebook connection, in this case you will be automatically connected by this service and you will find your account. If this is not the case and a pop-up asks you to connect to one of the services, please select Facebook, you will find your progress. 
- If you were logged in as a guest and had activated Game Center (on iOS) or Google Play Games (on Android), you will find your account by selecting the "Guest" option. Warning: you will now need to connect to another service in order not to lose your account.