Initramfs errors

This is a common problem. The symptoms are : the computer won't boot an the following shows up on the screen:

No init found. Try passing init= bootarg.

BusyBox v1.13.3 (Ubuntu 1:1.13.3-1ubuntu11) built-in shell (ash)
Enter 'help' for a list of built-in commands

(initramfs)_

Its caused by file corruption and is easily fixed by running a file system check:

(sudo) fsck -y /dev/sda1

You might be able to do this from busybox (a very limited rescue shell). Otherwise, use a rescue disc.