After trying to install the Gnome desktop in my Ubuntu 16.04 I couldn't boot again my computer after rebooting.
When I pressed ctrl+alt+f2 the message: "A start job is running for Hold until boot finishes up" appeared.
After a little Google searching I was able to figure out what the problem. It was a problem with Gdm. So I had to use again LightDM.
To fix it follow these steps:
- When booting press ESC key to enter in the Grub menu.
- Start Recovery mode.
- Select a Root session to be able to execute commands.
- Mount the filesystem in write mode again:
mount -o remount, rw
- Reconfigure lightdm :
After these steps your computer should boot again.