I've lost sudo access and I don't really understand why

[Tom H]

On Wed, Mar 18, 2020 at 9:57 PM Chris Green <cl at isbd.net> wrote:
>
> I'm running xubuntu 19.10 and I've somehow lost the ability to sudo to
> root.
>
> I *suspect* the cause was doing a usermod command to add my user to
> group vboxusers (so that VirtualBox USB works). Since I did that I get
> the following error when I try to do anything as root using sudo:-
>
> ris at esprimo$ sudo reboot
> [sudo] password for chris:
> Sorry, user chris is not allowed to execute '/usr/sbin/reboot' as root
> on esprimo.zbmc.eu.
> chris at esprimo$
>
> So how do I get my root access back (and at the same time not lose
> being in vboxusers for VirtualBox)?

You should've used

usermod -aG vboxusers chris
NOT
usermod -G vboxusers chris

You'll have to reboot in single-user mode (or reboot to the initramfs
shell, if you want to do more work) and run

usermod -aG sudo chris