it seems to be that Infinite light glitch in Poser13, sometimes it just turns off and you get a black render ! Often by moving the infinite light. It normally works again by moving the Light just a little and it turns back on. In some cases a really annoying issue. Also like in previous Poser versions the Spots keep on vanishing and you have a hard time locating these to move by hand even if you have em set a visible. So in Poser13 quiet some things seem not working stable concerning lights and the contrast change after a render, that can be awfully annoying.

so pretty much normal getting a black render in Poser13 with infinite light after they periodically crash ! just have to be getting used to it ....