Welcome to LQ!!! Can you 'talk' to grub (boot menu choices)?
Do you get a shell prompt? # _ dmesg/
journalctl cmd 'alive'?
Here's something for ideas on collecting info.
&1more
&a general wiki:
https://wiki.archlinux.org/index.php...roubleshooting
Here's a web-search, to gather ideas: fedora upgrade 20 to 21 failed|problems
(maybe add black screen, but a technical keyword on what broke would be ideal
)
Let us know a bit more details. Best wishes.