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I am using Linux CentOS but suddenly I couldn't run sudo command. I got below errors. Even with root user it got the same error.

$ sudo ls
sudo: unable to stat /etc/sudoers: Permission denied
sudo: no valid sudoers sources found, quitting
sudo: unable to initialize policy plugin
Below is the permission on this file:
 ls -l /etc/sudoers
-rwxr-xr-x 1 root root 4241 Jun  9 20:36 /etc/sudoers
# which sudo
/usr/bin/sudo
# ls -l /usr/bin/sudo
--ws-wx-wx. 1 root root 130712 Jun 10  2014 /usr/bin/sudo
What wrong with the system? How can I fix it?
EDIT1
Please see below command output:
# ls -Z /etc/sudoers
-r--r----- root root ?                                /etc/sudoers
I have run chmod 440 on this file but it seems the permission is broken.
EDIT2
I run below command to set the /etc/sudoers permission but it still not correct:
root@Cool-Too ~]# chmod 440 /etc/sudoers
[root@Cool-Too ~]# ll /etc/sudoers
-r--r----- 1 root root 4241 Aug 14 22:16 /etc/sudoers
[root@Cool-Too ~]# ll -Z /etc/sudoers
-r--r----- root root ?                                /etc/sudoers
                On CentOS, there are probably corrupted SELinux permissions.  Try restorecon /etc/sudoers  (as root).
– user4556274
                Aug 14, 2016 at 13:25
                Permissions are odd, they usually should be 440 for /etc/sudoers (i.e. -r--r----- root root). Use chmod 440 /etc/sudoers (as root) - if the problem persists use visudo and check if there are syntax errors in your sudoers file.
– FelixJN
                Aug 14, 2016 at 13:28
                Those permissions are really strange. Beware, even if you fix sudo, there may be other things that are broken. This doesn't happen on its own… Did you run strange commands recently?
– Gilles 'SO- stop being evil'
                Aug 14, 2016 at 20:59
-r--r----- 1 root root   4188 Mar 31 11:30 /etc/sudoers
---s--x--x 1 root root 130720 Mar 31 13:09 /usr/bin/sudo
To fix this, as root
chmod 4111 /usr/bin/sudo
chmod 440 /etc/sudoers
If you see a + sign in the ls -l output then it means there are also ACLs set; these should be removed
setfacl -b /usr/bin/sudo /etc/sudoers
(you don't seem to have this problem; I include it for completeness)
Since your permissions were broken, it's possible the SELinux labels were also broken.  You should verify the SELinux labels are correct with ls -Z:
-r--r-----. root root system_u:object_r:etc_t:s0       /etc/sudoers
---s--x--x. root root system_u:object_r:sudo_exec_t:s0 /usr/bin/sudo
These should be fixable with restorecon
restorecon -v /usr/bin/sudo /etc/sudoers
Finally you may have a totally problem install (eg PAM config, /etc/sudo.conf).  You can check to see what files have been modified:
rpm --verify sudo
If this throws up any files that look off then you might want to delete them and then reinstall sudo.
rm /etc/sudo.conf /etc/pam.d/sudo*
yum reinstall sudo
(This does require a working yum setup so don't do this if your repo's don't work!)
                I have tried these command but they don't work. I have updated my post for the output of ls -Z command.
– Joey Yi Zhao
                Aug 14, 2016 at 13:43
                OK, doesn't look like you're running SELinux (no labels) so that's possibly not your problem.  I don't see you having run the chmod 4111 /usr/bin/sudo.  I've also added additional things to check to the answer.
– Stephen Harris
                Aug 14, 2016 at 13:48
If sudo doesn't work, you can become a root user with su -
Later be root, you can re-install sudo using these commands below:
  
yum remove sudo
  
yum install sudo
You need to add yourself to /etc/sudoers file, enter:
  
visudo
Grant vivek user full permission via sudo:
  
vivek   ALL=(ALL) ALL
Save and close the file.
Later do that, you have sudo  and you can become a root user using:
  
sudo -i
If it doesn't work, use
  
sudo -s
                I become root and reinstalled sudo but it still not work. The output of sudo -i and sudo -s are the same as the error I showed on my post.
– Joey Yi Zhao
                Aug 14, 2016 at 13:48