This morning I tried to login as root in fedora machine it throws:

sh-3.00 # passwd root
passwd:unknown user name 'root'

I checked the /etc/passwd and /etc/shadow files and surpriswed to see the /etc/passwd.OLD file and shadow- file there.I deleted the file.
Now when I am trying to su - in runlevel 1 then its says user 'root' not found..though root user is already there.
--------------------------------------------------------------------
# useradd root
useradd:unknown gid 100
No group named "mail" exists,craeting mail spool with mode 0600
-----------------------------------------------------------------

I have very critical project running on this machine and unable to login in runlevel 3 or 5.

HElp???

You are much better with linux than I am, I think, but how about trying to login as an ordinary user and then opening a terminal and doing:

from there?

nothing if I am running the command:

sh # su <user>
user "<user> doent exist

and the same is for root too.

Sorry I am unable to help you. I noticed you also posted to some fedora-specific forums. Consider posting to justlinux.com if you have not already. They are some pretty sharp guys.

If I'm not mistaken, when in runlevel 1 you only need to type passwd and not passwd root. When in runlevel 1 you are root?

Type passwd to change the root password in run level 1

Please post your thread in only one forum. Posting a single thread in the most relevant forum will make it easier for members to help you and will keep the discussion in one place. This thread is being closed because it is a duplicate.

http://www.linuxquestions.org/questi...s-root-601813/

And another dupe (reported for closure) is here:
http://www.linuxquestions.org/questi...-login-602347/