HP PROBOOK 640 GI DISPLAY PORT refuses to work after clean OS install

HP has it's own propitiatory drivers. Usually the backup of all drivers is on the extra partition.
Meaning you would have to search your laptop specs and get the correct Mobile/Labtop drivers variant of your display.
Desktop may or may not work as well. I am a Thinkpad ( sadlly Lenova ) and in most labtops the display port ( VGA ) port
is often the over-ride/default video display option to the built in display. At least from my experience.

By not working have you tried to check that maybe it is not your port but maybe the display your trying to plug it into?

Another factor is that out of the box Windows since XP does not have a solution for using the max setting of any display at
all and are usually generic drivers meant to make things look nice at least.

Bottom line is that you need to find the drivers ( old or new ) and try to install from the earliest driver possible. If your unable to
install the driver correctly you need to "disable the "driver signing" because Windows 10 at a certain point do not understand
anything outside of installing via Windows update or whatever official updater that came with the installed OS device.

Meaning whatever driver you download that is past a certain point will work and should work and might actually give you a
false installation notice in device manager ( in fact in your device manager you should see what is installed for your GPU,
most Labtops are restricted to two GPU drivers being run at the same time based on drivers. However this could be limited to
one driver due to maker ), and check what is installed, you should also see your installed GPU on your Apps and features/Programs
and features by most chance if your installed from an installer package ( IDK about installing from the ugh app store ).

Point being is that newer drivers may have to "Disable integrity checks and enable driver test signing" before you attempt to install anything newer or later past a certain windows installation date. At least that is what I did to install my GPU with later drivers.
Again Windows 10 wants you to use Windows update ( or search online for new drivers ) to update the GPU in question.

If I was you ( to repeat myself ) I would install from the earliest driver up to the latest to see if the display port will stop functioning.
Maybe it will start functioning.

Only bad part is that some drivers do not have a downgrade function at all. Also if a driver unzips on to your drive you can go to the loaction zip the files and when you close out, go back to the location of your new zipped drivers and unzip them, then proceed to install selected features. However some features will not work while others will,