Hey there,
Since i'm new here and no one seemed to have answered your question...
Even though you seem to have solved this (like a boss) on your own, i still want to try and answer this.
Most of the time when shutting down the system during an update nothing will happen.
On reboot the update should be recognized as incomplete and should continue.
However if you shut down whilst updating an important file (system files or such) that file might get corrupted and your system will be unable to boot. or which is -in my opninion- worse it would cause "random" errors, crashes and slowness (in your case you would probably have weird graphical issues)
Shady