Look in the log (the yellow speech bubble in the far upper right). Is there a CUDA error? If that happens, no renders will work until you close and restart Poser. It will happen again at the same point if you re-render the exact same scene that triggered it originally, but it might not in a very similar scene. I find it happens rarely with Optix but never with non-Optix renders, so a regular GPU render is a workaround. Making sure all the characters are set to Unimesh may be helpful.

Studio driver helps too, as noted by Y-Phil.

They're still working on this one, it's much less frequent than it used to be, and it's now confined to Optix renders.