I ran into the same issue on my desktop with a primary monitor connected via DVI and a secondary by VGA, there was no output on the screen so I assumed the upgrade had hung.
In my case my system was still bootable but because package cleanup happens last I had a TON of "duplicate" packages installed and dnf didn't know how to clean them up so I cat'd the duplicates to a file (dnf repoquery --duplicated) and worked though the list in groups with "rpm -e --nodeps".
Finally got everything cleaned up... My media center computer is connected via HDMI and had no issues showing the status of the upgrade.