Sorry to bring up an old question but I had the same problem today, but the reason was actually because of the build order of the solution.
If you go into the Solution Property Pages -> Common Properties -> Startup Project.
The ensuring Multiple startup projects is selected, move the web project to the top of the list.
In my web application solution I have 2 web projects and 5 code projects, one of my code projects was at the top, and as such debug would not start.
Hope others find this helpful