[lubuntu] Black Screen after splash, but tty and recovery mode work

My Lubuntu 15.04 machine has a problem when booting.

When booting normally, it goes to a black screen after the LUbuntu splash screen. But if I press ALT+CTRL+F3 I get to tty and can log in (to a terminal!)

If I select a recovery mode, I can boot fine.

What steps can I take to fix this?

I tried ruing these from tty to fix it, but they have not worked:

sudo apt-get install --reinstall lubuntu-desktop, but it has not helped.
sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade