Probably a disabled installation here at your desktop.You could try to solve this with:
sudo apt-get update
then
sudo apt-get install --reinstall lightdm ubuntu-desktop
It can happen that there appears a query, which display-manager you want to set as main display-manager ---> so simply choose then lightdm.
Hope this helps. Here you can find more explanations about lightdm:
https://wiki.ubuntu.com/LightDM
And here more explanations about unity:
