If you recognize these symptoms then this article can help you :
- black screen at bootup
- bootup starts but halts
- bootup starts but halts and you see why – error message
- lightdm shows but is unresponsive
- lightdm shows – desktop is unreachable
When your system crashes, you might still be able to go into a TTY.
CTRL + ALT + F2, F3, F4, F5 and F6
Virtualbox : RIGHT-CTRL +F2, F3, F4, F5 and F6
TTY = TeleTYpewriter
Then retrace your steps.
Did you install something wrong?
Did you change the config?
Do you need to update or downgrade?
Use the alias rip to see what you installed last.
Use the power of arch-chroot.
If you can not use TTY to change your computer, this will be your last resort procedure.
It will get you back up and running. Tutorial is here.