I randomly found myself in a situation where I was able to boot up into Ubuntu but there was no way to log in. I could see my mouse. After doing some research, here is what I ended up doing to solve the problem.

• Get to the terminal by pressing Ctrl+Alt+F3

• Log in to the machine with your terminal

• Install GNOME

sudo apt install gnome

• Install the lightdm display manager

sudo apt install lightdm

• Reboot the machine