Try to access command line via recovery mode.
If you haven't changed your root password before, you can access it easily. If you have, you'll need to remember the password.
If you're on legacy (MBR):
Press SHIFT until you see the grub menu.
If you're on UEFI:
Press Esc until you see the grub menu.
Go to "Advanced options for Ubuntu" and select "Ubuntu with Linux-.... (recovery mode)". Wait until it boots up and select "root". It should ask you to give the root password. If you haven't changed it before, you can press CTRL+D to continue. Type the root password and try these commands to reinstall KDE Plasma and boot Kubuntu:
apt update
apt install -y kubuntu-desktop
systemctl reboot
You can also try to disable boot animation to see what's wrong.
vi /etc/default/grub
OR
nano /etc/default/grub
(use your preferred text editor)
Find this line:
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
and change it to this:
GRUB_CMDLINE_LINUX_DEFAULT="text"
Save the file and run this command:
update-grub
Reboot...
If you don't have root access in recovery mode or recovery mode doesn't boot up (ending up with the same error), then you'll need to boot from the live CD, chroot your Kubuntu installation and try those commands there.
I̲f̲ ̲y̲o̲u̲ ̲c̲a̲n̲'̲t̲ ̲r̲e̲c̲o̲v̲e̲r̲ ̲y̲o̲u̲r̲ ̲K̲u̲b̲u̲n̲t̲u̲ ̲i̲n̲s̲t̲a̲l̲l̲a̲t̲i̲o̲n̲,̲ ̲t̲h̲e̲n̲ ̲y̲o̲u̲ ̲c̲a̲n̲ ̲b̲a̲c̲k̲u̲p̲ ̲y̲o̲u̲r̲ ̲f̲i̲l̲e̲s̲ ̲f̲r̲o̲m̲ ̲t̲h̲e̲ ̲l̲i̲v̲e̲ ̲C̲D̲ ̲a̲n̲d̲ ̲r̲e̲i̲n̲s̲t̲a̲l̲l̲ ̲K̲u̲b̲u̲n̲t̲u̲.̲
If your bluetooth headset doesn't work again, check what's the cause of your issue. Don't reinstall audio drivers if you just can't connect to the headset.
Hope this helps! :)