This article explains what happens when a Trello workspace is deleted and how to recover the boards within it.
Workspaces cannot be restored
Deleting a workspace in Trello is permanent and cannot be undone. ProBackup does not back up workspaces as a separate data type, so a deleted workspace cannot be restored through ProBackup.
Boards are not deleted with the workspace
When you delete a workspace in Trello, the boards within it are not deleted. Instead, they are closed. You can recover these boards by following these steps:
Go to https://trello.com/my/boards.
Find the closed boards and reopen them.
Create a new workspace and move the reopened boards into it.
For more details, see Trello's guide on deleting a workspace.