Visual Studio F5 does not debug - Check if you have two projects opened simultaneously and one is awaiting for your response

Problem Scenario:

I try to debug a project in VS 2010. Build is successful, but F5/Start Debugging does not work. No error/warning. 


I have another project in VS2012, and it has a dialog box opened for an image file. But I do not find the file. So I want to search the web and seek an image... Then I open the project in VS2010, leaving the dialog box in VS2012 still open during the process.


Close the 2012 one.