[Solved] No top menu links on forum?
You've changed the active theme and wpForo menu was disconnected to the menu location. Please navigate in Dashboard to Appearance > Menus admin page. Select the wpForo Navigation, then check/enable it for wpForo Menu location.
Also please read wpForo Menu Doc:
For further wpForo core support please use wpforo.com. Here we mostly support wpForo paid add-ons.