"Username is not in the sudoers file. This incident will be reported."
In case this happens, you may have removed yourself from the corresponding group, or the some of the configuration files are otherwise broken.
You can regain root access at least as follows:
- boot the machine
- press `e` in grub boot menu
- add `init=/bin/sh` to linux boot parameters (the line starting `linux `)
- press Ctrl-x to boot
- get root prompt
Then you can add yourself back to the group or otherwise fix the files.