From the backup, restore the vars_forums.cgi file, as well as the Forum# directory for the deleted forum. Set wide permissions on the Forum# directory and its contents, then enter the control panel to make sure the forum exists and the settings are right. Afterwards, run all of the maintenance functions.
And in the future... be careful what you delete. <img src="https://www.ubbcentral.com/boards/images/graemlins/smile.gif" alt="" />