Mount /new_root: unknown filesystem type 'ext4'

post interrupted update is this disastrous trouble:

mount: /new_root: unknown filesystem type 'ext4'
You are now being dropped into an emergency shell.
sh: can't access tty; job control turned off

How, though lot googled posts was read and learnt, still nowhere to help ?

@abd That means the partition of your root dir is not ext4 and therefore it cannot be mounted. What did you do before, step by step ?