Hi Trevor. I'm Greg, awarded MVP for eleven years, Volunteer Moderator, and Independent Advisor here to help you until this is resolved.
- See if you can System Restore to a point before the problem started:
https://www.windowscentral.com/how-use-system-r...
System Restore is your first line of defense and can take you back to a point before a problem began. If it doesn't help then you can undo it. Read about it first in the tutorial above and then try restoring to just before this started.
If the problem repeats or System Restore doesn't work then check which Windows Updates were installed at Settings > Update & Security > Windows Updates > Update History, uninstall them there or System Restore again, then immediately check for and block the Updates with the Hide Updates tool explained here: https://www.tenforums.com/tutorials/8280-hide-s...
- Try these fixes for HDMI port not detected:
https://troubleshooter.xyz/wiki/fix-hdmi-port-n...
https://www.thewindowsclub.com/hdmi-port-not-wo... ((Avoid ad links which intrude into editorial copy, especially avoid Restoro ad)
https://www.windowstechit.com/16388/windows-10-...
https://jtechdigital.com/articles/how-to-troubl...
https://windowsreport.com/hdmi-output-problems-...
https://www.lifewire.com/fix-hdmi-connection-pr...
https://www.youtube.com/watch?v=kXTRIDvI8MU (Ignore ads)
- Try also these steps for known issue of second monitor not working:
https://www.lifewire.com/fix-second-monitor-tro...
https://www.thewindowsclub.com/windows-10-canno...
https://www.kapilarya.com/fix-windows-10-doesnt...
https://www.windowscentral.com/how-fix-your-sec...
- The Display driver is so important here that merely updating it is sometimes not enough. So let's try everything possible to get it working correctly:
First make sure you have updated the Display driver from the PC or Display adapter maker's Support Downloads web page for your exact model number, HP Serial Number or Dell Service Tag - from the sticker on the PC.
If necessary first remove the old driver using DDU (not necessary with Microsoft Basic driver which is a placeholder if nothing else is available) and/or installing in Safe Mode with Networking (so you have internet), or Safe Mode, accessed by one of these methods: https://www.digitalcitizen.life/4-ways-boot-saf...
While there check also for newer chipset, BIOS/UEFI firmware (very important), network, sound, USB3 and other drivers, comparing to the ones presently installed in Device Manager reached by right clicking the Start Menu.
If this doesn't give you the latest or ideal driver for what you need, compare it to the driver offered by the Intel driver update Utility here: http://www.intel.com/content/www/us/en/support/...
or the Nvidia Update utility here: http://www.nvidia.com/Download/Scan.aspx?lang=e...
or the AMD autodect utility here: http://support.amd.com/us/gpudownload/windows/P...
For Display issues one fix that is working is to Roll back or Uninstall the Driver on the Display Device > Driver tab, restart PC to reinstall driver.
You can also try older drivers in Device Manager > Display device > Driver tab > Update Driver > Browse > Let Me Pick.
Then you will know you've tried everything in addition to Windows Update drivers.
Adjust the screen resolution until it fits and looks best at Settings > System > Display.
I hope this helps. Based on what you report back I can answer any questions and may have other steps to perform.
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