Loss of old decks after downloading the app

After using Anki web and downloading the app on my MacBook,i have lost my old Anki decks .I have tried the backup method but doesn’t have my previous decks .How do I go about this please

To sync your data between AnkiWeb and the app, you must first log in with your AnkiWeb account.

I have tried that already and it didn’t work.

Could you be more specific about these points:

  • How (which platform, which Anki application) did you create your old decks;
  • What were the precise steps you took between having your old decks, and not having them anymore.