so I installed lxqt for trying it out and after removing everything back to normal, the only thing that remains is changing this login screen back to default one.
also when I try to power off, I get the message "other users are logged in and it shows my username.
Any help is appreciated.
When you installed LxQt it changed the display manager from the default GDM to SDDM, this was likely not uninstalled with the desktop.
Log back in to your desktop and open a terminal, type
sudo service sddm stop
to stop the current display manager, then
sudo dpkg-reconfigure gdm3
sudo service gdm3 start
to set the new (default) one.
Now you will have to reboot for this to take effect.