Just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command(s) below:
sudo nano /usr/share/lightdm/lightdm.conf.d/50-ubuntu.conf
Add the following line, save and close.
Reboot, and you're set.
Thanks to Gunnar Hjalmarsson
Based on Gunnar Hjalmarsson comment "ubuntu-session was last updated in the middle of March. Try sudo apt-get install --reinstall ubuntu-session" when that is done, the Guest session returns.
After some research, this can be done a different way as well.
/etc/lightdm/lightdm.conf file from your terminal using the following command:
sudo gedit /etc/lightdm/lightdm.conf
If the file exist, then just add the following line:
Otherwise copy and paste the following into it:
Save the file and exit the editor. Reboot your system, and the changes will remain even after an update to ubuntu-session as mentioned by Gunnar Hjalmarsson's comment.