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

I created an empty 'Demo' project in Visual Studio 2008 and added some existing projects to my solution. Included "MyHeader.h" (other project's header) in main.cpp file which is in 'Demo'. Also added header files' path in "Tools/Option/VC++ Directories/Include files" section. But intellisense says: "File MyHeader.h not found in current source file's directory or in build system paths..."

How the problem can be fixed?

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.

If it is the case that only the IDE indicates that it cannot find included files, but compiling is successful, the issue is simply that IntelliSense is not fully up to date with recent changes. This can happen specifically when including existing projects, in my own experience. Deleting the .sdf file (= IntelliSense database) that is generated in your solution directory forces Visual Studio to regenerate it, so that it is up to date again. Just doing a "clean" will probably do the same thing, but takes more time since everything will be generated again then.