can't login as root on a remote Redhat Server

I am currently working on a remote RedHat server(RHEL version 6.7 x86_64).
I can login as a user by using userID and password which is provided by

the server administrator.
But when I try to login as root by using command sudo su ,the console
gets hang infinitely.
To get out of this, I have to press ctrl+c key.
Also when I use su - root or su root, it shows incorrect password.
This was not the case some days ago.
Earlier I was able to login as root by using sudo su and then entering the same password provided by the server administrator.

To resolve this issue,I have done the following things:

1. Checked /etc/hosts, /etc/passwd, /etc/shadow, /etc/securetty files


2. Also checked /etc/pam.d/sudo and /etc/pam.d/su files


3. chmod 4755 /bin/su



4. chmod 644 /etc/securetty


5. checked root account expiry date(not expired)

What else do I need to do to resolve this issue?