In Visual Studio 2017, sometimes when you go to a previously saved file, the program does not open a window when you press F5, how to fix it?

Question

C++ / C#

in Visual Studio 2017, sometimes when you go to a previously saved file, the program does not open a window when you press F5, how to fix it?
i.e. I can't run the program

Answer 1

in Visual Studio 2017 (and indeed any) launching the program

window when pressing F5

available when opening a solution (*.sln is a way of maintaining projects in VS, it is more correct to always open solutions ) or a project (*.csproj, etc., in accordance with the language of the project, but in this case a temporary solution is also created , and when the project is closed it is suggested to save it)
can you open a single source file? (*.cs, *.c, *.cpp, etc.) in this case, compilation and execution are not available
ps somewhere quite recently, it caught my eye that supposedly the studio, starting from 2017, can execute individual C# files. Out of curiosity, I just double-checked - no, it does not. (perhaps it was about VS Code, but this is an independent product, with a bunch of its own features)