PC turns on but no display on monitor

[Monkee]

I've recently build a PC and it works fine: the fans spin, there are lights indicating power on GPU and motherboard but when I put an HDMI cord into the GPU there is no display.

 

I've reset the CMOS battery, tried a working GPU, changed the RAM slots and without RAM, replaced the thermal paste and tried a different PCIe slot for the GPU but nothing seems to work. It could be a PSU thing but the other GPU I tried needs no extra power so that should have displayed something right? I'm struggling to find anything else to try which is why I came here.

Please let me know if you have any more suggestions, I can send photos if need be.

 

My specs are:

-Z420 board

-XEON CPU E5-1620V2

-16GB RAM (ECC, Unbuffered)

-500W PSU

-RX 580