A few, just a few machines require a current BIOS. I see updates from late 2016 at https://support.hp.com/us-en/drivers/selfservice/hp-z420-workstation/5225033
Another common problem is of course drivers. But a proper PC with NO DRIVERS will default to some basic video mode to let us install the drivers.
Remember for W10 we always install in the order of motherboard, audio, video, then LAN, WLAN etc.
Another fine test is a boot to SAFE MODE which is documented on the web. Try that and get back to me.
Just installed a new 1050 ti into an hp z420 workstation with a six core Xeon at 3.2 GHz. It will show the windows 10 logo and that's it. It just stays on the logo. I uninstalled the old drivers to the old card before putting in the new one. I'm sure the connection is good and this card doesn't require any power connections. The PSU is at 600 watts so I feel like the gpu (which needs 75watts) should have enough power. Any tips? Thanks