ASP.NET MVC5 Access ViewBag from layout?

I would suggest that you have an action method, which the layout uses, that passes the data you need. For example

public class ControllerName : Controller
{
    public ActionMethod GetData()
    {
       return Content("Some data"); // Of whatever you need to return.
    }
}

then in the layout and the view you can call

@Html.Action("GetData", "ControllerName")