It sounds like you overwrote your old collection with a new mostly empty collection. A “streak” is just an add-on looking at the days you have review history in your collection, so if you get rid of that review history in your old collection, your streak goes too.
First, you’ll need to restore a backup from before the one-way sync. On desktop, there will be a backup that was created immediately before you did that. Backups - Anki Manual
But you can’t merge the 2 separate collections just by syncing – because the first sync will always be one-way. To merge them, you need to preserve one of them first, set up syncing, and then re-import it. Syncing with AnkiWeb - Anki Manual
If you are also trying to delete your old decks, that’s usually okay, because Anki will preserve their review history for stats (and streak) purposes. However, you can’t do that export/import-APKG-collection-merge process with that collection, because that old review history no longer belongs to those cards, and you’ll lose it.
You either need to merge the collections first, and then you can delete the decks/note you don’t want. Or you need to have the other newer collection be the one you export/import.