Game won't launch. Game Output and Launcher Log says, "Fatal error".

Put the summary of the bug you're having here

What I expected to happen was...:
I hit the play button, and go into my Minecraft map that I have been playing on for months now without any problems except for a little lag as usual.

What actually happened was...:
A few moments after I loaded into my world, I received a pop-up that said Minecraft was not responding. I exited out and went back into my world only to have it crash. The Game Output tab had a few things listed on it, so I looked to see where the game started to go wrong.
Right after it said "Looking for orphaned versions to clean up..." the next action said "Fatal error launching game."
The Launcher Log said the same thing, and told me to report it to this page. They said a bunch of other things as well, but I was a bit overwhelmed and was not sure how to fix it. Would appreciate some info. Thanks.

Steps to Reproduce:
1. Go to the Minecraft News/Launch page
2. Go into my account and hit play
3. Go into single player, and into one of my worlds, and just wait a few moments. (It pops up usually before two minutes)