If you've ever restarted your Lubuntu machine and been met with the error
"Xsession: unable to launch /usr/bin/startlubuntu Xsession /usr/bin/startlubuntu not found; falling back to default session"
after logging in, then you most likely accidentally deleted your Lubuntu install. Don't worry! Reinstalling Lubuntu is a breeze.
Reinstalling Lubuntu
While you're at the login screen, Press CTRL+ALT+F1. This will bring up a
terminal for you to work with. Log in with your credentials, and then execute
$ sudo apt-get install lubuntu-desktop
Then reboot your machine
$ sudo reboot
When you log in, everything should be operational now! Even your openbox configureations should still be preserved
How did this happen anyway?
Most likely an apt-get autoremove or an apt-get purge operation went
a little too far and actually deleted the Lubuntu package. Be careful next
time!