To reopen a board, first, click on the Boards button from the Trello homepage or directly from http://www.trello.com/my/boards.
From there, click on 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 can only reopen a board if you were an admin of the board.
When you click re-open, you can choose a different Workspace for that board.
Missing a board?
The Closed Boards drawer will only show the boards that you are a member of. Workspace-visible boards that you did not join will not appear in the list.
However, Premium Workspace Admins 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