Can't shutdown ubuntu 20.04

Everything was working fine, but suddenly i can't shutdown my ubuntu system. I've tried manually and from terminal but there is no difference. I also tried to change the grub file : sudo gedit etc/default/grub -made it so it looks like this- GRUB_CMDLINE_LINUX_DEFAULT="quiet splash acpi=force" but nothing changed.

I also tried GRUB_CMDLINE_LINUX_DEFAULT="" and trying to see which file blocks the shutdown after shutting down the system but in the end it showed Reached target Shutdown and immediately the screen turned black but the power was still on and the only way to shut it down was to press the power button for a long time.

After editing the etc/default/grub file i run the command sudo update-grub in both situations

The only thing i did was to change the root password to my system and then locking it again as it was by default. I don't know how this might affected my system.

Thank you in advance

==================================Edited====================================

The journalctl --system -b -1 command: https://pastebin.com/4J3zsxRg

From man bootup :

SYSTEM MANAGER SHUTDOWN
       System shutdown with systemd also consists of various target units with some minimal ordering structure applied:

                                             (conflicts with  (conflicts with
                                               all system     all file system
                                                services)     mounts, swaps,
                                                    |           cryptsetup/
                                                    |           veritysetup
                                                    |          devices, ...)
                                                    |                |
                                                    v                v
                                             shutdown.target    umount.target
                                                    |                |
                                                    \_______   ______/
                                                            \ /
                                                             v
                                                    (various low-level
                                                         services)
                                                             |
                                                             v
                                                       final.target
                                                             |
                       _____________________________________/ \_________________________________
                      /                         |                        |                      \
                      |                         |                        |                      |
                      v                         v                        v                      v
           systemd-reboot.service   systemd-poweroff.service   systemd-halt.service   systemd-kexec.service
                      |                         |                        |                      |
                      v                         v                        v                      v
               reboot.target             poweroff.target            halt.target           kexec.target

       Commonly used system shutdown targets are emphasized.

       Note that systemd-halt.service(8), systemd-reboot.service, systemd-poweroff.service and systemd-kexec.service will transition the system and server manager (PID 1) into the second phase of
       system shutdown (implemented in the systemd-shutdown binary), which will unmount any remaining file systems, kill any remaining processes and release any other remaining resources, in a
       simple and robust fashion, without taking any service or unit concept into account anymore. At that point, regular applications and resources are generally terminated and released already,
       the second phase hence operates only as safety net for everything that couldn't be stopped or released for some reason during the primary, unit-based shutdown phase described above.

Which means reaching shutdown.target isn't enough. Generally, poweroff.target is the goal. Maybe something is preventing a umount() ? Once you boot back your system, you should check the system journal of your previous boot, especially towards the end : journalctl --system -b -1

Did you try waiting for some time, say, 5-15 minutes, to see if it doesn't ultimately power off ?

EDIT : To get and share the full journal of the previous boot :

1. Type journalctl --system -b -1
2. Once it displays, I'm assuming the default less pager is used : type s (lowercase), it will ask you for a filename to save the full journal in the current directory, type a name of your choosing and press enter
3. Open the file with your preferred graphical text editor, mine is Emacs, but you may feel more comfortable with another one such as gedit
4. Select all text, copy, and paste where needed. This will probably be too big to paste in your question, hence my advice to paste to a service such as Pastebin

REEDIT : I should have noticed earlier that you mentioned you set a password for the root user. YOU'RE NOT SUPPOSED TO DO THIS. On Ubuntu, for security reasons, root shall not have a password, and you're supposed to reach root privilege through the sudo command from an admin user session.

While I suppose setting a password for root should not be the cause of your problem per se, I can't say I'm sure about what would happen if you open a session as root... maybe there's some Ubuntu idiosyncrasy that will pose problems upon shutdown.