I assume everything was fine one minute then the USB 3.0 ports just stopped working?
Have you tried to re-install the Texas Instruments USB 3.0 xHCI Controller driver? (on the HP Z620 driver download page).
It might also be worth a try re-installing the other motherboard chipset drivers.
The only time I have had any issue with my Z620 USB 3.0 ports was when I bought the machine earlier this year, the bottom-front USB port was physically damaged and caused random issues with other devices on the USB 3 ports on the rear. I ended up replacing the whole front USB assembly and no problems since. P.S. I knew the port was damaged and had priced the replacement part before the purchase.
Obviously, it would be better if it's down to a driver issue than a hardware problem.
