Hi,
was running Xenial & Mate just fine and then upgraded using: do-release-upgrade.
Now have no GUI. system is fine, if you want a headless box.
dpkg doesn't show any broken packages or errors. dmesg the same. I've only disabled the wireless interface, and am using ethernet instead.
The hardware is an old AsusTek X551MA laptop.
How to troubleshoot this?
Reboot with a live USB, rescue your personal data and rewrite the machine from scratch with a fresh copy of 18.04.
"Upgrades", in my experience, rarely work and trying to unpick what went wrong is often a rabbit hole with no end in sight.
cheers mate, was hoping to avoid this. I've months & years of python ML & docker integration work on this. box, but I reckon this makes the most sense. thanks again.
Sorry I could not be more positive in terms of fixing it. But, been there and done that... many many times.
I think Ubuntu should just drop promoting the "upgrade" service. It just doesn't work at least 50% of the time.
Maybe someone on here more knowledgeable than me could give more useful advice
I agree, should just not fall for the upgrade offer 
I used to fall for the bugger all the time....
I just rewrite from scratch now. Saves a lot of headache.