How do you resore deleted forum

[Reality Capture Experts]

How do you restore deleted forums

[opentype]

There is no function to do that. 

[AlexWright]

Yeah, your best bet here is to have a database backup.

[Joel R]

If you have delayed deletes turned on, then you should be able to restore your topics and posts for up to X days.  I believe the setting is in the Moderator Control Panel > Deleted items.  

In the future you may want to think about Archiving old forums instead of deleting them.