The command
sudo apt-get install --reinstall xserver-xorg-video-intel
or
sudo apt-get install xserver-xorg-video-intel
will work on 18.04 (Zorin 15) and on 20.04 (Zorin 16) as the drivers have been added to the Ubuntu Main Repository. The PPA is not needed.
I tried reinstalling first. The first command seemed to work but the second command returned the following:
sudo dpkg-reconfigure xserver-xorg
dpkg-query: package 'xserver-xorg' is not installed and no information is available
Use dpkg --info (= dpkg-deb --info) to examine archive files,
and dpkg --contents (= dpkg-deb --contents) to list their contents.
/usr/sbin/dpkg-reconfigure: xserver-xorg is not installed
what happens if you run
sudo apt-get install xserver-xorg-video-intel
and
sudo apt-get install xserver-xorg-core
Seemed to work, doing the next one now:
titude-D630:~$ sudo apt-get install xserver-xorg-video-intel
Reading package lists... Done
Building dependency tree
Reading state information... Done
xserver-xorg-video-intel is already the newest version (2:2.99.917+git20171229-1).
The following packages were automatically installed and are no longer required:
dkms libglu1-mesa libxatracker2 linux-headers-5.4.0-47-generic
linux-hwe-5.4-headers-5.4.0-47 linux-image-5.4.0-47-generic
linux-modules-5.4.0-47-generic linux-modules-extra-5.4.0-47-generic
python3-apport python3-click python3-colorama python3-problem-report
x11-apps x11-session-utils xinit xinput xserver-xorg-legacy-hwe-18.04
Use 'sudo apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
Also worked. No change in the screen though
atitude-D630:~$ sudo apt-get install xserver-xorg-core
Reading package lists... Done
Building dependency tree
Reading state information... Done
xserver-xorg-core is already the newest version (2:1.19.6-1ubuntu4.9).
The following packages were automatically installed and are no longer required:
dkms libglu1-mesa libxatracker2 linux-headers-5.4.0-47-generic
linux-hwe-5.4-headers-5.4.0-47 linux-image-5.4.0-47-generic
linux-modules-5.4.0-47-generic linux-modules-extra-5.4.0-47-generic
python3-apport python3-click python3-colorama python3-problem-report
x11-apps x11-session-utils xinit xinput xserver-xorg-legacy-hwe-18.04
Use 'sudo apt autoremove' to remove them.
0 upgraded, 0 newly installed, 0 to remove and 1 not upgraded.
Can you run uname -a and relay what kernel you are currently running?
Linux ruth-Latitude-D630 5.4.0-74-generic #83~18.04.1-Ubuntu SMP Tue May 11 16:01:00 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
Ah, ok good.
Can you run
sudo apt install python3-apport python3-click python3-colorama python3-problem-report x11-apps x11-session-utils xinit xinput
in order to get those to be set to Manually Installed.
Then
sudo apt full-upgrade
Reboot when complete.
I have a simple question.
Is it also possible to run
apt autoremove
instead of apt full-upgrade?
I can't log back into the computer after reboot. The password box is on the TV and it's not responding to mouse or keyboard.
Unplug your TV?
Certainly.
The Upgrade command is to just ensure all packages are up to date. Whether Hes is ready to run autoremove I leave entirely at his discretion...
Can you boot into the Recovery Menu - drop to Prompt and hit enter, then run:
sudo apt-get install xserver-xorg-input-all
I turned it off and did a hard shut down. No difference. 
While we are flailing, you might also try:
sudo apt install xserver-xorg-video-all
Sometimes power off the monitor is not enough. You need to physically disconnect it. (From my Raspberry pi experience).
Since his mouse and keyboard are not working on login; this tells me that the installation of xserver-xorg was the first installation, so input-all is a needed package to restore the mouse and keyboard functionality.
In the meantime... this is a really bad time for me to run away- but I actually have been lingering and am running late for an appointment...
Hopefully the installation of input-all will get Hes able to log in and operate normally for now and you and others can continue to help troubleshoot the flickering screen issue.
Unfortunately, it is midnight here in Paris and my concentration is at the limit. I can certainly continue on when I get up tomorrow morning.
Thank you so much for your help. I am pretty stuck right now but will see if I can get to a workable point while you both are away. No need to devote any more time if you don't hear from me that I got back to some decent point.
Sometimes not hearing back means the computer is a mess, too 
Long before internet, mobile phones and tablets, I quickly realized that I needed a computer to fix another computer. At each upgrade, I assembled a sub-machine from the leftover parts - which was awfully looked like my previous machine 