There are several issues that can cause that and they all have been covered through numerous threads in the forum. 

First I would check if the PoserSurface in the MatRooom has an active ambient channel, skin textures plugged into the Ambient_color with it's value set to one, or a very dark red color chip on same with it's value set to one. 

Second, your min shadow bias on your light might be higher than 0.1; another place to check.

Since you did not mention IDL in the render, I would rule out light emitter because it is a feature considered by IDL rendering.  However, it may not hurt turning that off as a third option. 

I am not going to bother mentioning the fourth since it is an IDL issue and, again, the OP did not mention IDL render.