Most likely, there is a problem with your code - the designer uses the whole code (myfile.cs and myfile.designer.cs) to "build" the designer window.
Close the solution, create a new one with just a default WinForms project and see if the designer appears.
If it does, you have done something silly with the code in your original project.
If it doesn't, then either the WinForms template is corrupt, or VS is.
You can try looking at the Winforms template file: Locate templates - Visual Studio | Microsoft Docs
] and perhaps fixing it (or finding a "pure" copy") but it may be easier to uninstall and reinstall VS.