A nested sequence is black .... why?

If you want people to help you you need to give more info: a nested sequence is black is not much to go on
What is the source material, seqeunce settings, effects used etc.
Did you use the option Nest or did you import a nested sequence into your timeline.

Assuming that you haven't started your nested sequence with a fade-up-from-black effect (and are not looking only at the first couple of frames), have you gone back to the original sequence to make sure that you have the video track enabled there? (On the far left side of the track, is the little eye being displayed?) If you had turned it off for some reason, it will appear as black video when you nest the sequence.

Was the sequence a multi-cam sequence? If so, did you have a blank video track selected?

This was a series of JPEG photos moving across the screen. When I nested it, everything looked fine. But, after rendering, it went black.

When images go black it usually means the dimension of the images is too large.