What happened to the code?

Amazon Web Services is moving code hosted at GitHub to a new GitHub organization: http://github.com/aws. Nothing is lost. Only moved.

What do I do now?

Simply update the URL of the repository's origin:

First, back up one commit:
$ git reset --hard HEAD^

Next, change then URL of origin:
$ git remote set-url origin git@github.com:aws/aws-sdk-net.git

Verify it worked
$ git remote -v

Pull the latest changes
$ git pull

Lastly, move your local repository to the new name (without the "for")
$ mv aws-sdk-for-net aws-sdk-net