Failed to start lightdm (stuck on bootloop)

Currently I was using kde on ubuntu. But after trying to install cinnamon and lightdm, I can’t start my OS. It is stucked on boot logo. And it’s showing, "failed to start lightdm". I tried to go to terminal using ctrl+alt+f3 but I can’t go to terminal.
Is there any way to change display manager by editing any configuration file?