Black screen after changing rate clock for external monitor

Question

As I have an external monitor with 144 Hz rate clock, I tried to set a new mode for it through xrandr. Nothing worked, however, then I noticed that in Gnome I can change the rate clock to 120 Hz. Unfortunately, after doing so, both laptop's and external screens went completely black. And after rebooting and logging in back to my account, the output just completely freezes. Though, Ctrl + Alt + F3 works.

I tried generating new config through xrandr. But I only get output Can't open display.
I also tried to move away ~/.config/monitors.xml as the system broke after tweaking settings through Gnome Control Center. That did not work at all.
Disconnecting the external monitor did not help either.
Additionally, reinstalling nVidia GPU drivers did nothing.
Lastly, running dconf reset -f /org/gnome seems not to have any effect either.

2 days later, I tried to reset Gnome settings thanks to this reply. However, if I try to log in, the system again freezes on the login screen.
Where are display settings located when even getting rid of all configurations does not help?

I am using Ubuntu 18.04 LTS on a laptop with Intel CPU (so an iGPU) with an eGPU nVidia.

UPDATE 1
After checking /var/log/Xorg.0.log, I found out this. It may be connected with me switching between tty and login screen but I am rather going to post it, it may be useful.

[   475.048] (II) system-logind: got pause for 13:72
[   586.128] (II) system-logind: got resume for 13:84
[ ... another resumes here ...]
[   586.318] (II) NVIDIA(0): Setting mode "NULL"

Answer 1

i just did what you’ve done several years before on ubuntu 22.04 using intel i912900k and nvidia rtx3090ti. Something lucky for me is that i had set ssh service so i can still using terminal by ssh connecting from another device. After reading your story i was a bit separated. however, i decided to find a way to fix the system and make everything back to work. it occurs to me that if i cannot deal with the problem by reset the fresh rate,maybe it is a way for me to reinstall xrog,nvidia driver or something related to the setting, and it could be solved while handling those dependencies. so first i uninstall everything related to the nvidia driver by using the command sudo dpkg -l |grep nvidia to see what should be remove,then using the command sudo apt install nvidia-driver-515.65.07_0ubuntu1, which the version relates to certain hardware. after that i found the problem is still there. then i tried another way of install nvidia driver and getting into the secure mode at of ubuntu to get the gui at last. nevertheless, something is still wrong. ubuntu can’t discover which nvidia hardware i was using and nvidia-smi also got weird output. i can’t train my dataset using cuda at the same time. so i download the driver 515.76 from nvidia official website and install it (got black screen temporarily here).after that the problem still remained. i wondered if the driver 515.65.07 still in dpkg and might be the reason so i uninstall everything related to nvidia in dpkg again and i got black screen for the second time! this time i reinstall the driver using the command ‘sudo apt install nvidia-driver-515 nvidia-dkms-515’ and following the steps in this link https://support.system76.com/articles/login-loop-ubuntu/ which for me is to move the old configuration files out of the way and everything became nice again.

Answer 2

I found out that no solution on the Internet really worked; some stuff even got worse when GUI stopped working completely even after reinstalling it. And as nobody has replied here, I have a simple solution.

If you get to the same issue as I got, the only working solution for me was reinstalling the system. However, as data in /home can be kept, it is not such a huge issue (even when the installation software does not look like it would like to keep the data).

Anyway, I am not going to mark this as the best answer. Maybe somebody in the future faces the same issue and finds some working solution. Or the person can do the same as I did; start from the beginning again.