I changed the permissions of the sudoers file to
640 to write to the file.
But now I am unable to use the sudo command on my machine. Whenever I try to use sudo, I get the following o/p-
the permissions of sudoers file are 0640, should be 0440.
Please help.

Do you have a live CD or USB? If so, you can chroot into your installation and change the permissions.

If you know the root password for your system, you can escalate to root via 'su -' and change permissions that way.

Otherwise, I think an easier solution would be for you to boot into the server via single-user-mode by passing the option to GRUB at start-up (google it), dropping you into the shell as root, and you can go on from there:

At the grub boot screen:

1. Press e to edit an entry
2. Select the line starting with the word kernel
3. Press e to edit the line
4. Append the number 1 to the end of the line
5. Press the [Enter] key
6. Press b to boot

Log in as root (i.e username root).

You should always state what Linux version you are using. Different distros have different rules for root and sudo behavior.

You can skip all of that by selecting the recovery mode entry.

And to the OP, you should use visudo to edit your sudoers file.

I am using Ubuntu 9.10. So I cannot login as root. There is no root account.
I also cannot su to root. I get "Authentication failure"

What happens? I see the login screen. I have to again login using my normal account.

I checked the man page of visudo. visudo is for editing the sudoers file.
How can I change the permissions of the file to 0440.

There is ALWAYS a root account.

Isn't that what you wanted to do as you mentioned:
Why else would you want to write to the sudoer's file, if you did not intend to edit it?

First, from the GRUB boot menu, there will be two entries for a specific kernel release. There's the first entry that boots directly into multi-user mode and directly beneath it, there's the recovery mode entry that boots into recovery mode or runlevel 1.

So for instance, if your kernel release is: 2.6.27.4, your GRUB menu will have:
Select the second entry (recovery mode). This will automatically log you in as root and take you directly into the command-line. Note that Ubuntu does not set the root password by default, so entering runlevel 1 or recovery mode will never ask for root's passwords.

Now change the permission of your sudoer's file to 0440:
And reboot:
Hope this explains it a little better.

Oh, forgot about that. I'm used to having "fallback mode" instead.

Is the login screen a virtual terminal or graphical?

Fallback and recovery modes are just different names for the same thing. They both start the sytem in single-user mode or runlevel 1.

On Arch, fallback just uses a different ramfs image. It doesn't change the runlevel.

Login as root and run "chmod 0440 /etc/sudoers"

True, but the default install of Ubuntu is to disable it and automatically give the first registered user full rights via sudo instead... sigh ..

I did it from the recovery mode. Could change the permissions of the sudoers file as root.
But isn't it unsecure to get the access to root account from the recovery mode without a password?
But Thanks all for the help.