Visual Studio 2010 Professional Play Button Disabled

I'm having problems with one of my projects in VS2010 Pro, it won't allow me to hit the "debug" play button at the top of the screen, it's been disabled, when I open other projects or start new ones it's enabled on the new project / other project, however, it's disabled on the one project, I don't know what I changed (it's been awhile since I opened this as it pissed me off last time I opened it), what can I do to re-enable the "play" button? (debug), this is a Windows Phone 7 project and I was messing with the .csproj code. Edit: The menu-items have been disabled "Start Debugging: F5" doesn't work, as well. Thank you.