Yosemite Upgrade : All user accounts lost admin or administrator privilege in mac, What you will do?

When does this happen?
Sometimes it happens when you upgrade your OS version, or it happen when you try to delete or alter privileges of an user account. Or sometimes trying to give same account name for two accounts.

What you will do when you lost admin or administrator privilege on your mac on all your user account?
When you are in such situation, you will never be able to install anything or delete from your mac because you don't have any account with Admin privileges.

To get yourself in such situation you can follow the steps below which worked for me.

1) Restart your PC and while starting up press and hold Command + S key
2) Login into an standard account will land up in an bash terminal.
3) Execute "fsck -fy", and wait until you get your drive is OK.
4) Execute "mount -uw /"
5) Execute "rm /var/db/.AppleSetupDone", to delete the file so that Apple will allow you to create an admin account on reboot.
6) Execute "exit" to to reboot.

On Reboot apple will allow you to create an admin user account.

Hope it helps...