iPhone or Android?

I think in general there are better solutions for syncing notes than syncing to your Gmail account. Might look into that if you get this resolved.

The way I understand notes in Gmail (something that I don't do) is it creates a folder called notes when you set your phone to sync with your gmail account for your phone notes. So when you delete the notes folder in your gmail account, you lose all the notes on your phone as well.

The only advice I could give would be to log on to Gmail on your computer and look in your trash. Google kinda hides the trash from you. On the left side under you inbox there are several other categories. One of them says "more labels". Click on that and then you should see your trash.

Hopefully once you click on trash you will see that folder in there and you'll be able to recover it. Good luck.