If you go to the menu File -> Switch Profile and hit the Open Backup button, can you find an earlier backup of your collection? Restoring an Automatic Backup (Recovering from Data Loss)
If you have your cards on iPhone, you can sync them with Ankiweb by selecting sync and choosing upload (on your iPhone), later on, your computer you can select sync and choose download to download your collection from Ankiweb.
In the future, you can try to keep your own backup your collection (your Anki directory on your local computer), see Managing Files and Your Collection. In my computer, I daily run a script that backups the whole Anki directory daily up to the last 100 copies/days.
Related threads:
- https://anki.tenderapp.com/discussions/ankidesktop/41315-how-to-recover-deleted-flashcards
- https://anki.tenderapp.com/discussions/ankidesktop/41464-deleted-cards
- https://anki.tenderapp.com/discussions/ankidesktop/41527-error-solution
- https://anki.tenderapp.com/discussions/ankidesktop/41713-uploaded-old-version-of-decks-to-anki
- https://anki.tenderapp.com/discussions/ankidesktop/41746-lost-front-definition-on-cards
- https://anki.tenderapp.com/discussions/ankidesktop/41957-deleted-profile-on-anki
- https://anki.tenderapp.com/discussions/ankidesktop/41920-deleted-deck