Normally I would have recommended running the onboard diagnostics but Apple messed it up under Mojave so I don’t think it will ofter anything useful. Restarting the system and pressing the '''D''' key (see the reference above). To rule out a bad GPU you’ll need to visit an Apple Store as their diagnostics still work, go figure! ;-{
'''The other approach:'''
Without knowing via diagnostics you’ll need to take the long path here hoping you just have a OS corruption. You’ll need to create a bootable OS installer thumb drive so you can see if you can boot under it.
While possible to do in within your Windows partition I would recommend using a Mac as its easier and quicker. Here’s the easy steps:
First you’ll need a clean image download the image from here: [https://support.apple.com/en-us/HT210190|How to upgrade to macOS Mojave]. Jump down to Step 4 for the link. Apples certificates expired on anything older [https://appleinsider.com/articles/19/10/24/if-youve-got-an-old-macos-install-image-it-will-probably-stop-working-today|If you've got an old macOS install image, it will probably stop working today]
Next, follow this guide: [https://www.macworld.com/article/3284378/how-to-create-a-bootable-macos-mojave-installer-drive.html|How to create a bootable macOS Mojave installer drive]
To do this within Windows here’s how: [https://www.intowindows.com/guide-to-prepare-bootable-usb-of-mac-os-x-on-windows-pc/|Prepare Bootable USB Of macOS Mojave On Windows 10 PC]