When you have changed from Legacy to UEFI, you will have made the PC unable to boot at all.
The fans will spin, but that's about all. Correct? No windows startup music or anything?
It sounds bad, but it's not scary.
All you'll need to do is reset the BIOS.
This does involve removing the bottom cover, and there is either a reset bios button, or pair of pins.
You take the laptop battery out, open the bottom of the laptop, and find the "Button Cell" (Round shape battery, like you would find in a wrist watch), which you then also unplug.
Wait 10 minutes, then reconnect it all, and power on.
This is the most reset your computer can possibly get.
Which is good.
I manage a fleet of a few hundred HP Probook 450's.
Is there more to that Driver_Failure_State erro you haven't typed?
Did you get a photo of the error screen?
It normally means the Driver is working (the screen is on) but it's entirely incorrect \ wrong.
This happens because Windows10 does it's best effort, at guessing a driver that will "Just Work".
What you need, is to find a way to boot cleanly, possibly Safe Mode, or VGA mode, and install your computers SPECIFIC driver.
From either the HP website, or the NVIDIA\AMD\INTEL webpage, depending whos Graphics Chip you have in there.
If you can get it back to the stage where it boots, but fails at the Failure_State error, and there is no way to continue with ANY option (safe mode, etc).
You'll need to look into how to "Integrate Drivers with Windows 10 Install" so that when it 'guesses' the driver, the correct one is there.
Hope that helps!