Sometimes it happens, that you have created folders in Visual Studio for your application and had put some *.cs C# Source Code files that suddenly disappeard in the solution explorer.
I.e. that can happen when you have forgotten to save your Visual Studio application project, berfore you have closed the IDE.
Use the file explorer and navigate to your Visual Studio solution (alternatively you can right-click a source code file in your folder and select „Open contained folder“ from the context menu).
Open the .csproj file in your solution in a text editor of your choice:
Go to the section where the <ItemGroup /> tags are declared and add a new section with the folders and source code files you want to show in your solution like in the screenshot below
You can add all cs-files with a wildcard star like it is shown in the screenshot above.