/script ChatFrame2:Show()
Apparently they botched something in the latest patch, and excessive use of the map (like when you're gathering) can cause it to flake out and disappear.
Put the above in a macro and you're good to go.
Combat window disappearing in WoW? (Found this solution)
Combat window disappearing in WoW? (Found this solution)
I ain't leaving without your soul and I am sober this time.