Debugger lost his process
It's happen a couple of times recently. The debugger refused to start, unable to find the process attached to my web project, obviously aspnet_wp.exe.
Just as an advice for beginners, if you are sure that the Debugging mode was working fro you before, rather than trying to change every settings, just quit your project and ... restart your PC ;-)
It's often the simplest thing to do, and of course if it's happen again use the Help button on the Error box next time to investigate further your issue.
I still searching the origin of this (bad) behavior. Not sure about the source, maybe the aspnet process.
Thankfully, this happened rarely but if you look at the Help regarding this error, the list of things to diagnose is a bit scary.