This kind of error is a really hard one to find, it can be many things. To me it sounds like a problem with the kernel itself, maybe the update manager downloaded another kernel update, and the package may have been corrupted.
This error can first appear when you boot the operating system, the kernel needs a ramdisk ('initrd' usually compiled into the kernel, or as a seperate file initrd.gz or initrd.img), to mount under itself (kernel filesystem) during boot up. If there is a problem with the initrd, then the kernel will not boot and will give this error.
It may be a possibility that there is a problem with the /dev directory, there are a few things in this directory that the operating system needs to boot, /dev/ramdisk /dev/console ...etc, it might be worth looking in here as well, the update manager may have updated soemthing it shouldn't have.