Plus the fact the GeForce GPU unit is its illuminated bright green, so it's obviously plugged into the MB or it wouldn't be illuminated.

This post is puzzling. Did you upgrade from 16 to 32 or did you add 32 to 32 for 64 GB total?

XPS 8900

4.01 GHz

32GB running, then added 32 GB

Windows 10 64bit

Put the original RAM alone back in and see if the system boots. If so then put the new RAM in alone and try booting. If that is successful then try all the RAM but before you do that check the GPU to make sure it is fully secured in the PCIe slot. It sounds like during the removal of the GPU support the card may have partially pulled out of the MB.

Not possible if the GPU is the GeForce bracket, that has been securely positioned back in the MB. In any case, the monitor would show a 'no connection' sign - as it is it shows a black screen. Will try the Ram modules again tomorrow.

I added 32 to bring me up to 64.

1st attempt - after fitting new Ram modules, black screen, pilot light on boot switch flashing amber.

Removing new modules, the pc booted normally.

2nd attempt - after removing & refitting Ram modules, after removing GeForce bracket, then securing everything back in place:- black screen, pilot light no longer flashing.

You cannot manipulate the Ram tabs by the GeForce bracket, it is hard up against them, that's why I needed to remove the bracket. All this sweat just to install new Ram.

Notice also the pc will now boot on replacing power cable, don't know why. When I push the power button to turn the pc off, it goes off, then 5 seconds later reboots itself.

I've had to unplug it or it will just stay on in black screen mode.

Hopefully this will be diagnostic.

Thanks.

Thanks. Will try again tomorrow, it is late here.

This all means I will have to remove the GeForce bracket again, multiple times - I cannot access the RAM slots with the bracket in place. 

@kr236rk02  - You are connecting the monitor to the add-in video card, and not to either the onboard Intel UHD Graphics HDMI or DP port, correct?

The fact that the system booted normally with the new RAM removed sounds like RAM that is not compatible. 

The monitor is connected to the GeForce GPU.

May have unbuffered Ram instead of buffered.

The crucial Ram recommended for my XPS was not available - it was DIMM, what I have is UDIMM.

If I have erred - is there anywhere I can get the correct Ram for this pc please?

Many thanks.

Update:

Removed the GeForce bracket - which is in the form of a girder the length of the pc, after disconnecting it from the monitor. The bracket itself has to be unplugged further along its length.

Then I could get at the Ram-retaining (white) catches which are otherwise hard up against the bracket. I released the new Ram modules & removed them, leaving the old twin 16GB modules in the black slots.

Replaced the GeForce bracket & reconnected the monitor.

Plugged the pc back in - it made a whirring sound then stopped, which is usual when the power supply has been removed then reconnected. I booted the pc manually with the on/off button on the tower. The pc booted normally, full screen graphics, the lot, as if nothing had ever happened.

So it's the new modules, or the slots - this pc has never had all 4 slots going at the same time.

The white slots are now empty.

What I ordered -

2 of Crucial RAM CT16G4DFRA266 16GB DDR4 2666 MHz CL19 Desktop Memory
ECC Type = Non-ECC, Form Factor = UDIMM, Pin Count = 288-pin,
PC Speed = PC4-21300, Voltage = 1.2V, Rank and Configuration = 1Rx8 or 2Rx8

In place of this, which I identified but couldn't find available:-

Crucial CT2K16G4DFD8266 32 GB Kit (16 GBx2)
(DDR4, 2666 MT/s, PC4-21300, Dual Rank x8, DIMM, 288-Pin) Memory.

Could this be the problem please?