Follow these steps if you want to restore any template that has been deleted:

1 - Go to Conversational Channels and select the WhatsApp line or FB Messenger account where you want to retrieve the template(s).

2 - Once inside the account or line, click on "Templates":

Swipe to the bottom right of the screen and click "Deleted Templates", then hit the restore icon: ↺

That's it! Now you will see the template available along with the others.