Since another monitor doesn't work it is probably a failed video card. Remove the card and try using the integrated video to test. You must actually remove the video card from the computer to use the integrated video.

https://topics-cdn.dell.com/pdf/xps-8930-desktop_setup-guide_en-us.pdf

Make sure you are connecting the monitor correctly to the video card. Use the bottom slots on the back, not the ones on the upper middle left. The integrated video doesn't work when a video card is installed.

Service manual-- https://topics-cdn.dell.com/pdf/xps-8930-desktop_service-manual_en-us.pdf

Many thanks for your advice, it’s much appreciated, you were right - the graphics card was loose! I’ve now got the screen working which is great but... When I restart the computer it gets stuck on Support Assist, it says ‘Hardware scan complete with no issues’ but when I press continue it goes back in to support assist scanning again for potential hardware problems. I ran diagnostics which reported ‘all tests past’ but still I can’t get out of Support Assist, any ideas?

Or you can try:

1. right-click on Start, click on Task Manager, click on the Support Assist task, and then click End task.

2. right-click on Start, click on Run, type services.msc, click OK, scroll down the list of services until you find Support Assist, then double click on it, and change the Startup type to Disabled and then click Apply and OK. 

After restarting, you can uninstall Support Assist.

Many thanks for your advice, it turned out to be a loose video card. I’ve got the screen working, which is great Many thanks again

Many thanks for your advice, it turned out to be a loose video card. I’ve got the screen working, which is great, but I can’t login now! Dell Support told me to update the BIOS and now I’m stuck on the blue screen of death - 

‘Stop code: PAGE FAULT IN NONPAGED AREA. What failed: iaStoreAC.sys’

so I can’t get in to safe mode

AAAHHHH!

If you can't boot into Windows or Safe Mode or you have not created a recovery disk/media in advance then you may have to use Windows installation media to boot and repair. First, you have to download and create the Windows installation media. Be sure it is the same version as installed on your computer (i.e. Windows 10 version 1909). See this article: https://www.thewindowsclub.com/boot-or-repair-windows-10-using-the-installation-media/

There is also a way to use the Dell SupportAssist OS Recovery Tool to recover/repair. See this video: https://www.youtube.com/watch?v=_cVIuJAUCNU

Hi, many thanks for your advice. I created the recovery media on a USB flash drive but when I click on it in the boot list the machine crashes! Dell tech support suggested I update the BIOS and that’s now causing a blue screen crash, the situation seems to be getting worse!

Many thanks for your advice it is appreciated but my PC starts loading support assist and then crashes, I can’t even get in to support assist!