Small warning: Ubuntu's "do you want to upgrade" popup window defaults to "yes". Had to find that out the hard way when a user hit Enter the wrong moment and was suddenly sitting in front of a bricked system.
Usually you press Enter to accept things and ESC to cancel. Also, it first downloads all packages before upgrading, so you should've had time to cancel it.
I had a similar experience upgrading my laptop(though I initiated the upgrade on purpose). Just crapped out, but an apt install -f followed by a dist-upgrade fixed it all and it seems to have worked.
