How do I un-do a deleted block?

@AlexS - Sorry this happened to you!  Unfortunately, I don’t think there’s an “undo” function in Klaviyo (yet).  For what it’s worth, one of the workflow processes that I try to do is to always “Clone” a template before I begin working on it, or periodically “Clone” it save I always have a working backup copy.  This doesn’t help you here, I’m afraid, but a good tip to keep in mind for the future.

Here’s how to do it:
 

Klaviyo Documentation:

• How to Create, Edit, Delete, and Manage Templates → Clone or Delete a Template

Hope this helps for next time.