Symptom: Unable to attach to the process. Catastrophic failure.
Resolution: Change project settings to enable edit and continue, then press F5 to start the web project in debug mode.
I also deleted my solution.suo file from the solutions directory as it was over 3MB! - more about that here -http://merrickchaffer.blogspot.co.uk/2012/02/visual-studio-hanging-running-slowly.html