Yosemite Install or post-install Boot-up Frozen

Smartphone typing, please excuse brevity :)

I'd attach a photo, but can't seem to.

The screen I'm on does not have a time remaining estimate, that was the prior screen.

This one simply shows the apple logo and a plain status bar with no percentage or time indicator.

Perhaps it's just the boot up screen.

At any rate, it's been stuck exactly at the halfway point for about an hour.

No clue what the proper course of action here is.

It's a 15 in MacBook Pro, mid 2009, Latest version of Snow Leopard with no history of install related or hardware issues at all.

Thanks in advance!