I have the same problem. I’m working in the CitySample and made a duplicate of the Small City Level to work in. The original level is fine, but after months of work in my duplicate level (making a short film - over 160 Level Sequence shots) - it now is totally black! The only light source is the sm_sun in the far horizon and a few street lights. Any point light I introduce does not work.
I’ve tried copying over settings from the original small city level - without success. It does not seem to be a “project” based problem - only a level based problem. Fortunately, one solution is to load the original small city level and open the Level Sequences in that, but I still need to reimport (copy/paste) a bunch of assets.