HP Z420 build wont boot.

[OMGRBG]

Just finished a simple gaming build with a used hp z420 motherboard. Turned it on for the first time so I could go to the BIOS and install Windows 10, everything seemed to work perfect until I noticed a solid blue LED on my motherboard, and that I was recieving no output to my keyboard, mouse, or monitor. I looked into it on google and tried a few things like resetting the CMOS, and using different RAM that I was positive worked, but nothing. Any tips? (The PSU, and GPU work fine in another one of my PCs)

[DGroves]

are you using the z420 power supply? if not that's most likely your problem as the z420 supply is not a ATX pinout and connecting a ATX supply will directly short out the supply/motherboard and possibly damage the board

there are adapters to convert the HP supply to ATX,

https://www.amazon.com/BLUEXIN-Power-Adapter-Workstation-12inch/dp/B07Q9HWYRY

[OMGRBG]

I am using a EVGA 600W psu which has a 24 pin powere connector, but I use an adapter that converts it to the Z420 workstation 18 pin motherboard connector.

[DGroves]

is the adapter specifically made for the z420? not just any 18 pin adapter will work

[OMGRBG]

The adapter is designed to work with the z420, z620, and z230. 

[DGroves]

remove all pci-e cards including video, disconnect all hard drives, usb devices you want just the motherboard powered

now power on does it start?

if so install video and connect GPU aux power leads (from the EVGA supply in your case) 

does it start and display video?