I had this exact same problem. Everything was working perfectly, and then suddenly the Pi wouldn't boot.
I finally found a monitor and keyboard for it (I had been running it headless for weeks) and found myself in this emergency kernel mode, despite there being nothing wrong with my root partition.
I figured out the renaming trick a few minutes before finding this post, but I too would like to know why the pi started doing this in the first place. There isn't a lot of documentation out there.
Interestingly, it seems that now, after getting back to the main kernel, my network card no longer works, so I wonder if there is some strange hardware problem that's causing the emergency boot.