After purging wingpanel and then installing it again, I rebooted and logged in. The login interface disappeared as usual but nothing else happened there was just the background image. I had this problem before and I fixed it by reinstalling pantheon (at least I think that's what I did, can't remember) however it doesn't seem to be working now (ctrl+alt+f2)
Fixed it by removing and install pantheon.
sudo apt-get remove pantheon sudo apt-get install pantheon sudo reboot
Try to return Cerbere to default:
gsettings set org.pantheon.cerbere monitored-processes ['wingpanel', 'plank', 'slingshot-launcher --silent']