Three HP ZBook Ultra G1a's all updated to Windows 11 25H2 yesterday and refused to progress past the boot screen.
The workaround patch for this issue is to long press the power button until it does a full reset, (20 seconds or so) and then get into the bios. Note that when you reconnect to power you should also connect a USB to Ethernet adapter.
This worked on 2 of the 3 affected devices I have.
On the third one, the only way to get into bios was to crack open the laptop, pull the battery, long press power for 20+ seconds, then reconnect.
Either way, once it lets you into the bios, go to Update System Bios > Check HP.Com for BIOS updates.
When you select this, the laptop goes black and reboots. You will then need to repeat the same process you used to get into the bios to get into the bios again, only this time it will tell you it needs to disable BDE if you have bitlocker enabled.
Then it will allow you to roll back the bios to 1.03.11. Do so.
Once the rollback is completed the laptop will boot normally.
I have reported this issue to HP Support, but wanted to share in case anyone else is dealing with the same issue. If it had only been 1 device I wouldn't have cared, but for 3 to get hit with the exact same issue at the exact same time when the exact same update with the exact same bios was applied, I figured that it would be useful for more people to know about it.
Hopefully this is somewhat helpful!