Visual Studio 'Run' button is disabled, or problems with build

Sometime very strange things happens with Visual Studio. The one have happened to me maybe three times for all time, and each time I already forgot that I need to do to fix it! So basically your simptoms can be any of the like:

1) Your Run (F5) button is disabled;
2) Your program is not recompiled and hitting Run causes running old version of the 'exe' that lies in the Debug folder;
3) Then hitting F5 you receive error messages like "Visual Studio cannot start debugging because the debug target 'C:\your\path\bin\Debug\Project.exe' is missing. Please build the project and retry, or set the OutputPath and AssemblyName properties appropriately to point at the correct location for the target assembly.'

That usually that happens absolutely sporadically with you doing nothnig with the project or solution preferences, which confuses a lot. Now, in 99% of the cases the solution is to go to the Build->Configuration Manager... and check that Build checkbox is checked for required projects, see picture:



What makes them suddenly unchecked is still a mistery to me.