$ sudo -i
[sudo] password for rohara: 
rohara is not in the sudoers file.  This incident will be reported.

This was surprising to me after rebooting my Ubuntu system, it was working beforehand.

No need to fret! If you have the root password just become root another way:

su

Then you can modify /etc/sudoers to add yourself to the file. You can copy the root line and change root for your own username.

This is now my /etc/sudoers file:

# User privilege specification
root	ALL=(ALL:ALL) ALL
rohara  ALL=(ALL:ALL) ALL