I have been having this problem multiple times, and there are so many non-working solutions online. So I thought I should recorded for future usage.

Edit the following file with this content:

sudo vim /etc/lightdm/lightdm.conf

vim can be replace with whatever command-line editor you have available, nano, emac, etc…

[SeatDefaults]

greeter-session=unity-greeter

greeter-show-manual-login=true