The message is probably what you typed in the close board section in the CP.
Have you tried opening the board?
Maybe the conversion failed or got stuck so it never completed.
With that said your database is subject to be corrupted.
Either case you probably need to reload a database backup.
The script normally should have closed the board converted the database then opened the board again.
There is not a undo option.
When you say not open to guests I assume that it is not open to normal users either because normally when a forum is closed it is only open to a admin.
