Could you try rebuilding NodeBB? Perhaps something happend with just this step and redoing it will fix the forum.
If this doesn't work, then reinstallation will probably be required (or at least the easiest way).
Viewing my user page here in the nodeBB community, I encounter the following problem when clicking on the hamburger button.
As one can see, the menu pops up perfectly fine, but is rendered left of the button, where the visible browser window ends (I moved the full size browser window a bit to the right so that the problem can be seen more clearly).
So far I have seen the problem on this nodeBB instance only, which is partly because my own instance does not show the hamburger button at all at the moment (see this related topic).