I tested to link 2 desks to see what’s.
Then, I make sure one got 300+ cards and the other stayed empty.
When deleting the empty one to break the link and 'cause it doesn’t matter anymore to keep it, I lost Everything… All the 2 desks are gone.
How can I undo it ? Or how can I retrieve it… ?
I’m not sure what you mean with linked decks, but you can try this:
https://faqs.ankiweb.net/restoring-an-automatic-backup-recovering-from-data-loss.html
You can delete notes from the browser, but not cards; deleting a note deletes all its cards. If you want you can edit the note type so it doesn’t produce certain cards (if you do so, don’t forget to delete the empty cards). It is also possible to individually choose whether to produce a card or not using conditional replacement.