Cannot boot system due to start job running for hold

You can fix this in following way, it worked for me. FYI : I had upgraded from 14.04 to 16.04.

Go in your safe mode by hitting SHIFT key or ESC. select networking , then drop to root prompt. Then run following commands.

sudo apt-get remove plymouth
sudo apt-get remove xserver-xorg-video-intel

Now, reboot,

When you reboot, still you may get black or purple screen,

Press Ctrl+Alt+F1 and login with your username and password.

Then run following commands.

GDM ie Gnome display manager was also causing problems for me, so following step was essential for me

I switched to lightdm from gdm3 as display manager.

sudo apt-get install lightdm
sudo dpkg-reconfigure lightdm

When you are prompted, select default display manager as lightdm

Now I was back with desktop , without dash or unity but only icons and wallpaper. now run this.

sudo apt-get install ubuntu-desktop

Now, reboot after this.

The desktop is back!

now, you can install intel graphic drivers again,

sudo apt-get install xserver-xorg-video-intel

Reboot now. Its done!

Had the same problem earlier today.

Found out here (https://bbs.archlinux.org/viewtopic.php?id=195223) that could be a plymouth related problem, and after removing it (I'm really not into looping logins animations anyway...), I got into trouble with nvidia drivers.

I just removed all nvidia related software and, voilà, I'm here typing this answer.

sudo apt-get remove plymouth
sudo apt-get purge nvidia
sudo reboot

For those who messed up with Linux booting after the update (Which happened to many users including me) Here's the complete solution (Which I used, and I was able to get back everything).

1. Don't panic!
2. If your computer is hanged at the login screen, press Ctrl+Alt+f2
3. It will ask for your login username and password.
4. Connect ethernet cable (LAN cable) from your wifi router or modem to your computer
5. Type dhclient eth0 and press enter
6. For testing networking, try ping www.google.com if there's any failure then please check your LAN connection, router or modem (Connecting to WiFi is a tedious process so try LAN connection first)
7. Type reboot and press enter.
8. Now keep pressing Esc or Shift key.
9. Enter into latest kernel recovery mode.
10. Use keyboard arrow keys to navigate.
11. Press Enter on Networking
12. Now it will display the previous screen.
13. Click on root (You need root access to modify system).

14. Run

    sudo apt-get install -f --reinstall lightdm
    

    (It will download 200mb of data)

15. Run

    sudo apt-get install unity
    

16. Run

    sudo apt-get install ubuntu-session
    

17. Run

    sudo dpkg-reconfigure lightdm
    

18. It will ask you to select one out of two options: gdm & lightdm. Select lightdm

19. Run

    sudo apt-get install gnome
    

    ( It will download 700mb of data)

20. Run reboot and you should be able to get your work back (Although your desktop will look ugly because of conflicting themes. Don't worry, steps 23 & 24 will fix this issue).

21. Open terminal (Ctrl+Alt+T) and run

    sudo apt-get autoremove
    

    (to purge all unnecessary dependencies which can cause problems while installing new packages)

22. Run

    sudo apt install unity-tweak-tool
    

23. Run

    sudo apt-get autoremove --purge light-themes
    

24. Run

    sudo apt-get install light-themes
    

25. Finally download ubuntu-tweak from ubuntuupdates.com (version that works with Xenial Xerus and isn't available anywhere else!)
26. Reboot your system and done!