If I try to open any of my Administration task as a normal user (System -> Administration -> firewall or samba or date & time or SELinux management or display ......etc) I'm getting a error message window saying "insufficient rights". This is happening last two days only, Earlier i used to access all the above mentioned tasks. My Normal user is in sudoers list as well. SO help me to come out of this problem.

Two days back i lost the sudo capability, then i tried the following commands
# chwon root:root /usr/bin/sudo
# chmod 4755 /usr/bin/sudo
# reboot

I doubt, whether this may doing some problem

Hard to tell what you've done. It's also hard to tell what distro you have because you haven't told us.

If you have a root user, you can log in as root, or switch to superuser via "su" command if you've hosed the sudo command.

Check man sudo for information on editing the sudoers list.