If you have deleted a quiz, it can be recovered within 30 days. After 30 days, it'll be permanently deleted from your account.
To recover a quiz, go to your Quizzes tab and go to Trash:
Select the quiz you'd like to restore and then click Restore:
The quiz will show up in your quiz list again! You have the option to Delete Forever here as well. Keep in mind, delete forever is a permanent action and cannot be undone. If you have any deleted folders, they'll also show up here and can be restored the same way.