You can logon from a tty/console (CTRL+ALT+F3 through F6). From there you can check the /var/log/dpkg.log. If it's still processing, perhaps leave it.
If you ran the upgrade using the graphical "Update Manager" then you may see it busy in "top" on the terminal. If you used do-release-upgrade then you may see that.
If you did use do-release-upgrade, then you may be able to "attach" to is with the screen -r to reconnect to it. However that's not possible if you used "Update Manager".
Alternatively kill the upgrade that's running and then immediately dpkg --configure -a which will finish off whatever apt package upgrades were in flight. This is usually sufficient to complete the upgrade, by carrying on where it left off when you suspended it.
If you do choose this path, make sure you kill the right things. Don’t reboot again until you get past killing the processes and re-running dpkg.
Find the update-manager process:
ps aux | grep update-manager | grep -v grep
If there’s a line of output like this:
alan 3269811 0.0 0.0 9076 2200 pts/2 S+ 21:33 0:00 /usr/bin/update-manager
Take the number from the second column (in my case 3269811, yours will differ), and use it to kill the process:
sudo kill -9 3269811
I’d also check for any dpkg processes using:
ps aux | grep dpkg | grep -v grep
Again, if there is one running, we can kill it with the same process as above.
Now try and finish off the package upgrades with:
sudo dpkg --configure -a
In theory, you will likely see some packages whizz by as they’re updated.
If that works (as in, runs to completion), then you may want to just do one last sudo apt update ; sudo apt full-upgrade or sudo apt install ubuntu-desktop^ in case anything went missing in the meantime. Then reboot and cross your fingers ;)
