Mr.J.
Without any display and error notices, except the fan not running leads me to think of BIOS trouble, although you tried restting CMOS which didn't help. It's difficult to narrow the prospective problem.
However, the fan not running may be the trouble. When I was upgrading a z620 I didn't seat the CPU/ memory shroud properly. Consequently, the memory fans were not getting power and the system would not start. Servers and workstations are very protective of potential thermal stress.
First check that the shroud is seated properly so the little 6? pin connector is fully seated, and see what happens.
If that doesn't improve the situation, as you are starting from a blind point, consider resetting the crisis jumpers to normal and again clearing CMOS. I installed the 3.90 BIOS in a z620 and the start up became very slow and it flashed though 8 or 10 screens of what appeared to be three of four copies of BIOS. Clearing CMOS restored it to business as usual.
If you can see the BIOS screen on restart, check that the factory default settings are restored.
If you have further problems, try running the system on a Windows installation disk or WinPE. Also, consider loading the original OEM Windows 7 Professional which you can do at no cost and which will activate automatically.
Cheers,
BambiBoomZ
HP z620 (Rev 2) 2X Xeon E5-2690 (8-core @ 2.9 /3.8GHz) / 40GB DDR3-1600 ECC) / Quadro K2200 (4GB) / HP Z Turbo Drive (256GB) / 800W > Windows 7 Professional 64-bit > HP 2711x (27" 1980 X 1080)
[ Passmark System Rating= 5322 / CPU= 19675 / 2D= 767 / 3D = 3544/ Mem =2337 / Disk = 12951 ] 8.15.16