Omg, thanks for the "Change Path" option! Just totally saved me.
On that note, I think what happened is that my images were so deep inside folders inside folders that Photoscan got confused. Although it went as far as aligning all the cameras, but then when I went to build a dense point cloud, it couldn't find any images (and I hadn't moved them). Anyway, I then did move them all higher up in the directory structure, and used the "Change Path" on all images, and it seems all good.
Thanks!
- Jance