After restarting my laptop which run Kubuntu 15.04 beta, it got stuck at a black screen with a message: Starting version 219. I tried to reboot it a few times, and it always displayed the same message. Fortunately, I found a solution from this page which fixed the problem.
Here’s a recap on how to do it.
1. When you see the above message, hit Ctrl + Alt + F2
2. Run sudo systemctl enable sddm.service -f
3. Reboot it
So the issue seems to lie with sddm, a login display manager, not wanting to start up.