Quite a few days back,I faced a peculiar problem :).Visual Studio was not having the green play button(the one for Start Debugging) enabled.No way was I able to start debugging.
Google gave many suggestions,none was of help.
I soon found out that,in Startup Projects(From the menu Project -> Properties),the option multiple was selected and all the projects were set to an action None)(Setting this to Start Without debugging also creates this same problem) :)
Yet to find out how this happened automatically but still setting up the startup project,made the green button glow :)
Be sure to check this the next time if the green button doesn't glow :)