Try cleaning the cache in lightmass if you haven’t already. That might help. If lightmass crashes or hangs. The first thing you do is cleaning the cache. I’m pretty sure it’s a lightmass thing and not a general unreal problem.
If cleaning the cache doesn’t work then try building level by level (if you have sublevels) to find out which level is that causing a problem.
Or go delete half of your meshes and try to build and then try to build the other half until you find the asset that causes the problem. It can be hideous.
We had someone here scaling a high polygon bed throw (300.000 verts) down to 0.01 so he could use it as a tissue. That caused lightmass to crash surprisingly 
- What I am trying to say is that lightmass does work in general. So you may need to find out which of your assets causing it to not work for this scene. If it works in other scenes for you.