What do you mean with "Lubuntu requires an external screen"? That sounds very odd to me. A screen is not required to run Ubuntu. In fact, many servers don't have one, Raspberry Pis don't have one, etc. Do you simply mean that your internal screen stopped working on Ubuntu? If yes, I would suggest you simply say so - then this has nothing to do with your external screen, so talking about your external screen is simply confusing. If no, can you specify what you mean with "Lubuntu requires an external screen"?

The fact that the fonts are tiny on your external screen is likely due to some incorrect system settings and (very) likely unrelated to the fact that your internal screen is not working. It is not clear which of the two problems your are trying to solve here. If you want to solve both, you should open two separate questions (possibly linking one in the other) to keep concerns separate.

@MalteSkoruppa Yes, you're right about the wording. Sorry about that. Fixed now. They both happened at the same time though, so I'm pretty sure they're related. I wouldn't be surprised at all if the same solution fixes both.

Anything useful in /var/log/Xorg.0.log? Btw, if you can't read anything on your current setup due to your problem, I would suggest to connect to it from another computer using ssh if you have the possibility. (e.g., maybe you have another spare laptop, a tablet, or something like that). That could make debugging a lot easier. :)

@MalteSkoruppa Possibly, but even after trimming what I thought was redundant (multiple copies of the same report block with the same info), I still get Body is limited to 30000 characters; you entered 38266. There's a bunch in there about the sleep button, touchscreen, touchpad, etc., so how much is actually relevant? Can I trim that out too to post here?

Normally, I do have a second laptop with the same system installed on it, and SSH already set up to do exactly what you suggest. But I'm away from home for another week or two, and I didn't know to bring it with me.

You can use pastebin.ubuntu.com.

@MalteSkoruppa Oh! I never knew about that, yet somehow I have an account already. I'm pretty sure I never signed up for Ubuntu One either. Anyway: pastebin.ubuntu.com/p/24tSjCHdmW

Hmm, looks like X does not see your internal monitor at all. 🤔 What is the output of xrandr --listmonitors and xrandr --listproviders?

@MalteSkoruppa pastebin.ubuntu.com/p/M4myb9qkYq (copy/pasted both ways because I really can't see much)

Ok, what you can see from the --listproviders command is that your Intel GPU isn't recognized at all: Ubuntu only sees the NVIDIA GPU. I would speculate that somehow the drivers for your Intel GPU are not working anymore (e.g., due to a kernel update) and the internal monitor is connected to that GPU and thus it is not working. That also explains why it works in the BIOS and in Windows. To confirm that suspicion, can you boot an Ubuntu live system from a USB stick, see if that works with your internal monitor, and same output as above (/var/log/Xorg.0.log and the two xrandr commands).

I meant: "..and post the same output as above", so that we can compare. :) If the Intel drivers are working on a live system, we should see the Intel GPU there and the monitor in the /var/log/Xorg.0.log, and then we have determined the cause already.

Ok, now we know for sure it's a driver problem. What's interesting is that on the live system, only the Intel GPU is recognized and not the Nvidia GPU - exactly the opposite basically as in your normal system. For the live system it's possibly just a matter of installing the Nvidia drivers (open or proprietary). For the normal system it's a bit odd, because Intel should run out-of-the-box, but perhaps there is some weird interference with your Nvidia GPU. I assume you have one of those fancy hybrid graphics card thingys where you can basically choose which GPU should be used.

Unfortunately, I have no experience with those hybrid graphics cards, but I assume the solution would lie in understanding your current driver configuration. Are you using the open-source or the proprietary Nvidia drivers? Also, I think these hybrid graphics cards can be managed under Linux with a technology called PRIME. What do these commands output: glxinfo | grep OpenGL and DRI_PRIME=1 glxinfo | grep OpenGL? Does this question (with its answers) help you? askubuntu.com/questions/661922/…

@MalteSkoruppa pastebin.ubuntu.com/p/SC2PfqY7Xk

I thought I was using the proprietary driver, but Lubuntu's Additional Drivers says it's manually installed. I must have done something with that at some point, well before it stopped working. cat ./.bash_history | grep nvidia returns one line: sudo apt install nvidia-driver-430

I thought that question looked promising, but as you can see from the pastebin, nvidia-prime was already installed. My NVIDIA X Server Settings still doesn't have the PRIME Profiles tab.

Hmm, I wonder if these proprietary Nvidia drivers and the PRIME package are not the problem in the first place. But let's focus on the Intel driver first, since this is what seems to not be working for you. It would be interesting to see if your system can see the Intel GPU, and whether you have the Intel drivers installed (usually, they should be installed). Can you post the output of the following commands: (1) lspci; (2) lshw -C video; and (3) apt policy xserver-xorg-video-intel. Thanks!

@MalteSkoruppa pastebin.ubuntu.com/p/YSpKFcCY6V

It's odd, it seems like your system does not see the Intel GPU at all, yet the Intel drivers for X are installed. I wonder if you somehow uninstalled the Mesa drivers so your system can't communicate with the Intel GPU. Could you post the output of dpkg -l | grep mesa? Also, just out of curiosity, what is the output of prime-select query?

@MalteSkoruppa I don't remember doing that. I think I would if I did. Anyway: pastebin.ubuntu.com/p/f36FMfVxdk