Visual Studio can't 'see' my included header files

Delete the .sdf file that is in your solution directory. It's just the Intellisense database, and Visual Studio will recreate it the next time you open that solution. This db can get corrupted and cause the IDE to not be able to find things, and since the compiler generates this information for itself on the fly, it wouldn't be affected.

If you choose Project and then All Files in the menu, all files should be displayed in the Solution Explorer that are physically in your project map, but not (yet) included in your project. If you right click on the file you want to add in the Solution Explorer, you can include it.

This happened to me just now, after shutting down and restarting the computer. Eventually I realised that the architecture had somehow been changed to ARM from x64.