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

[Chris Green]

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)?

-- 
Chris Green