XPS 8700, won’t boot

Hi there.

I recently built a new PC to replace my old

Dell one.

the plan was to use the old dell for home assistant.

I’ve gone to try the PC today and it won’t boot.

The fans all run, but no display output and no power to mouse / keyboard.

Have tried:

removing BIOS battery 

2.9v

Used test button on power supply, all ok.

Ram checks ( tried each stick on its own individual y.)

Removed GPU and plugged monitor into Motherboard.

Coming up at a loss now.

if I leave the PC to run it power cycles ever 5mins or so.

Before leaving it for a few weeks it was running with no faults. 

Thanks in advance 

Chris