I'm not sure I understand, but I'll make a guess.
Was this an ASP.NET project in VS2003? Did it have .Designer.cs files in it? Is it the case that you've done an upgrade but you find that you have no .Designer.cs files now?
If so - you may have inadvertently changed your Web Application Project into a Web Site "project". This sort of "project" was introduced in VS2005, so you would not have seen it yet. These aren't real projects (no .csproj file), and they have many other differences.
Otherwise, if your project didn't used to have .Designer.cs files, then why do you want them now?
In general, the way to create designer files is to open the file in a designer, wait a few moments for the designer to finish what it's doing (updating the screen, etc); and then saving the file. You might possibly have to "touch" the original in order to get the designer to reevaluate things.