Humm...
Could it be some topic management has occurred. such as moving topics,deleting topics, adding forums blab,Bla.
I would start with the rebuild content in the control panel and or clear cache while you are there.

Control Panel » Content Rebuilder
I would run all of them
It might take a while on a very large forum.

Then
Control Panel » Clear Cache

Or try any combo in between.


Blue Man Group
There is no such thing as stupid questions. Just stupid answers