I can't help right now because I'm working with two authors, but don't give up hope on your reader. Things happen -- accidents, illness, computer blowup.
I'm basically in the same boat because, all of a sudden, one of my betas has fallen silent. He had finished commenting on my novel but we were still working on his. We became good friends while we exchanged chapters. Often we mailed each other with silly things like jokes, or how our day was going, but suddenly - silence. The last I heard he was about to move furniture and heavy stuff into a new apartment. I hope he didn't hurt himself or damage his computer, but until I hear I'm worried.
I guess I've been lucky because most of the betas I've worked with have turned into long-distance friends. I hope you hear from yours soon.