Baked light using “Build → Build Lights Only” and everything became dark, but it was light before baking. How can I fix this?
Hi!
It’s hard to tell… I can’t see a thing in your scene! 
Before baking you see the scene if it was lit dynamically…
Do you have a second uv map for each object for lightmaps?
Ok so before building lights if you place objects in your scene they get a standard dynamically lit look… but they won’t look realistic because of the lack of GI… If you use static or stationary lights then you’ll have to build your lights and your static meshes will get GI information too…
Every mesh has to have a uv map for (baked)lighting information! Even if you don’t have textures! Imagine it as a “lighting texture”. Without it Unreal won’t be able to visualize the lighting for you!
Hi! Before baking scene had two light sources and it was light on it. Not sure what is uv map, I don’t use textures, only materials with simple colors
Just switch your light’s mobility from Static to other modes. It triggers’s lighting system to update.