Reopening a closed board
To reopen a board, first, click on the Boards button from the Trello homepage or directly from http://www.trello.com/my/boards.
Choose the ‘View all closed boards' button. You may need to scroll to the bottom of the page if you have a lot of boards.
Find the board and click " Re-open".
You will need to have been a Board Admin to reopen a board.
You can choose a different Workspace by clicking on the drop-down menu with your Workspace's name.
Missing a board?
The Closed Boards drawer will only show the boards that you are a member of, and Workspace-Visible boards that you did not join will not show on the list.
However, Premium Workspace Admin can reopen a Workspace board they're not a member of by entering the board link directly in their address bar to see the 'Closed Board' page and an option to re-open the board.