Comment 1 for bug 43517

Could it be that you ran out of space in the /boot directory?
I use a separate boot partition, and it is too small, so now each time I need to update the kernel I must first move the init ram disk (which takes most of the space), otherwise I'm left with an unbootable state (apt deosn't get the error message from gunzip, and reports no installation error).
Always have a livecd at hand, or store a working kernel / etc. so that you can boot it manually in grub.