lost root access

[Jack McGee]

I had a user on a 18.04 machine that had root access, using sudo, but it 
lost it.  This just between reboots.
> jmcgee at jmcgee-desktop:~$ ssh -l mythuser 192.168.1.105
> mythuser at 192.168.1.105's password: 
> Welcome to Ubuntu 18.04.1 LTS (GNU/Linux 4.15.0-42-generic x86_64)
> 
>  * Documentation:  https://help.ubuntu.com
>  * Management:     https://landscape.canonical.com
>  * Support:        https://ubuntu.com/advantage
> 
>  * MicroK8s is Kubernetes in a snap. Made by devs for devs.
>    One quick install on a workstation, VM, or appliance.
> 
>    - https://bit.ly/microk8s
> 
>  * Full K8s GPU support is now available!
> 
>    - https://blog.ubuntu.com/2018/12/10/using-gpgpus-with-kubernetes
> 
> 
>  * Canonical Livepatch is available for installation.
>    - Reduce system reboots and improve kernel security. Activate at:
>      https://ubuntu.com/livepatch
> 
> 13 packages can be updated.
> 12 updates are security updates.
> 
> 
> You have packages from the Hardware Enablement Stack (HWE) installed that
> are going out of support on 2023-04-30.
>         
> There is a graphics stack installed on this system. An upgrade to a
> configuration supported for the full lifetime of the LTS will become
> available on 2020-07-21 and can be installed by running 'update-manager'
> in the Dash.
>     
> Last login: Sun Jan  6 14:19:31 2019 from 192.168.1.64
> mythuser at amethi:~$ sudo ls
> [sudo] password for mythuser: 
> mythuser is not in the sudoers file.  This incident will be reported.

I rebooted and not change.
I tried this:
https://help.ubuntu.com/community/LostPassword

but I don't see a Grub menu, just drops to XFCE desktop after bios 
splash.  Holding down shift key.

How I get root access back?