if we change "root" to something else, how do we change it back?
 

accidently in /etc/passwd, i changed root to oot...
how do i change it back?

LiveCD

Perhaps you linux distro has a rescue option in the boot menu? If that's the case, boot into it and lauch an editor to edit your /etc/passwd file.

there is a recovery option in the boot menu...
how do we proceed?

in boot menu, we wrote "single" on the next line, using grub edit ...
after that the normal boot started with no change in the problem...

If you have LiveCD or rescueCD from your distributions, use that one. There is also init= boot parameter hack, but I would use it only as last option.