Ubuntu 20.04 not loading GUI?

Question

linux

Greetings!
Ubuntu 20.04, BIOS boots, UBUNTU splash screen loads with animation, black screen appears with a blinking bar in the corner, then nothing.
I tried the key combinations Alt + Ctrl + F3 ... F6, it seems that a console appears asking for a password, but after a second it disappears, nothing can be entered. Alt+Ctrl+F2, Alt+Ctrl+F1, Alt+Ctrl+F7, etc. don't work at all, just like Alt+Ctrl+F, Alt+Ctrl+T, Alt+Ctrl+Del. The only way to exit is by pressing the power button. Now I'm writing from Windows, which is installed on another disk.
There is no alternative bootloader, because it is not needed - the OSes are installed on different physical disks and the boot is controlled in the BIOS. There is an Ubuntu Live CD. There is already quite a lot of data on the Ubuntu disk that you don't want to lose. What to do? How to diagnose and fix this?

Answer 1

in the grub boot menu (if the machine rebooted by reset, then the menu will appear the next time it is turned on, well, or press esc in the first second of boot) select the recovery mode, enter the root password (I hope they did it, it was for the root user otherwise you will have to boot livecd into chroot and write a password, although in this mode you can do everything without resque), then enter the command:
journalctl -xf
will issue a system error log, use the arrows to view, study what does not work
. Also, during the boot of ubuntu, when the logo is shown and progress is running, you can press esc (or tab, maybe I'm confusing or both work) and the current download log will open, at the moment of freezing, you can see what happened.
If the black screen still appears at the right time, you can turn off the logo completely, for this you need to press 'e' in the grub boot menu and edit the text config file of the selected boot menu item, removing quiet splash wherever you find it (this is the linux /boot kernel settings line /vmlinuz....) and run with the edited f10 settings, then the log will be in text mode (it helps if there is a problem with the video card drivers or with the video card itself)
, in general, until you diagnose the problem, you cannot solve it.
ps the data is easily copied by booting from livecd, in most cases a copy of /home is enough (make the whole backup anyway) which can be deployed to a new installation (of course, install the packages manually, you will also need machine configurations, but they can be copied from the backup), and everything will work

Answer 2

scan the disk for errors,
this is corrected in 80%.