I’m about at my whits end. Tried an older driver - 570, and now neither monitor works. No signal to either!
One more idea. I haven’t brought it up yet, because it initially worked for you and your OP says:
I was under the impression that the installer always chooses the latest HWE stack, and you seem to have used the latest point release (24.04.4), which should have resulted in kernel version 6.17 being installed. Did you select 6.8 manually? Or am I misunderstanding this:
Desktop installations of 24.04 default to tracking the HWE stack.
So maybe just try manually installing the HWE stack as per that guide.
In a nutshell:
- stick to X11 for now to eliminate Wayland issues
- install HWE stack
- reboot
If it’s still the same issue, try different older nvidia drivers, starting from the the last before 580.
BTW, the Noveau driver of Linux 6.17 might work better with your GPU.
Try booting in recovery mode.
The only thing I could do is run the USB installer and run in TRY mode. Can’t get into the system any other way. But of course in this mode, My graphics are scrambled so I can’t access the installed drivers screen. I want to go back to the recommended driver 580, so at least my computer runs with one screen. I have a lot of work to do and do behind. Sorry, I’ve been so patient all week, but I’m ready to say enough is enough.
I’ll try recovery mode😞
Don’t worry, it’s just a more basic mode of operation, to at least get you a somewhat tolerable graphics output.
Oh, one very important thing I’ve just remembered. A black screen can also mean that some system component is failing and the system service manager (systemd) is waiting for it. To make sure that isn’t the case, you should wait at least five minutes after booting up. I don’t know the exact timeout, but if by then you don’t have some emergency shell, just press the power button (briefly) to try an initiate a clean shutdown. Give that five minutes grace period as well.
It won’t work. Tried the shift key. Tried the escape key. I’m dead in the water. Think the only thing I can do is scream and install Ubuntu all over again. 
Try hitting Esc repeatedly as soon as the boot splash image of your UEFI disappears. The time window is very short at times.
Hitting escape took me into HPs start up menu. I can’t seem to get to Grub. Will try again, then giving up for the evening.
Yeah, I feel you. I don’t know what they were thinking when landing on Esc for that. 
Holding down shift worked fine with BIOS, whereas I’ve run into Esc being issued too early quite a lot of times myself.
The usual recommendation is to increase the grub menu timeout, but for that you need a working display first.
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You could also try the UEFI boot menu; F2 or F12 should work. Then select “ubuntu” (or maybe “grub”?) and after hitting Enter immediately hit Esc.
I’ll give it a try tomorrow. Ms Optimistic has given up tonight. Thought I’d never say this, but just about ready to go back to It, who should not be mentioned… Sigh! 
Don’t even think that! 
I think, since you were pondering a fresh installation already, how about checking out the just released Ubuntu 26.04? It says in the release notes that GNOME Shell only runs on Wayland now and that Nvidia drivers should play a lot nicer with it.
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I think I need to buy you dinner.
Darn it, I will try that. I’m the type of person who may get discouraged once in a blue moon, but I’m not known to give in. After all, I used to repair Windows for customers. Oh no, I spoke the W word. See, you made me smile! That’s a good way to end the evening. Now for some game time on the phone. Good night Peter. 
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I am catching up on the changes with 26.04 right now and I stumbled upon this:
I think you should make sure to tick that box, or whatever it may be – haven’t seen it myself yet. And if the installer runs in garbled graphics mode again, which I hope it won’t, you can also start it in Safe Graphics mode. But apparently there were some changes to the boot menu of the installer, so I can only point you to this dated tutorial, to get the idea and find your own way once there. 
If you cannot find any option that says “Safe Graphics” or similar, you should be able to edit the command line and just append nomodeset.
P.S.: Or, since 26.04 comes with the Linux 7.0 kernel, maybe you won’t even need the proprietary Nvidia driver. If the installer, which runs with the open source Noveau driver, IIRC, exhibits no graphics issues, you could give that a try. Depending on your needs, it might just be good enough. I’ve also heard talk of Nvidia developing their own Nova driver as a mainline kernel module, but I cannot say I know much about it’s current status. But I am sure Canonical has evaluated it and if they’ve found it viable, it should install automatically.
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So far, so good this morning. I installed 26.04 and right now everything is working as should. Wow, two monitors, no garbled text. Now I just have to reinstall all my programs. No big deal. I’ll let you know if I run into any problems later in the day. But, right now, things are awfully sunny.
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Make sure to check for updates. I’ve often found that even with an online installation there are some newer packages right after first boot. Then do a reboot, to check if everything is working. Only then you should start with software installation.
Good advice as always.
Well, I updated and rebooted and much to my delight, everything works. Many thanks to both of my helpers, with special thanks to Peter. It was a struggle, but we have arrived. I’m happy to be an active member of the Ubuntu family. I also know where to come for future assistance. 
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