If you were trying to build the Android source with the instructions present here, and after
repo syncstage, when you reboot the machine and it doesn't get past the splash screen, you will need to reinstall the ubuntu-desktop again. As stupid as it sounds, I spent almost a day trying to tweak VirtualBox settings with no luck. So, go ahead and just do the following:
sudo apt-get install --reinstall e.g.
sudo apt-get install --reinstall xubuntu-desktopsudo apt-get install --reinstall ubuntu-desktopsudo apt-get install --reinstall kubuntu-desktop