What kinds of modifications did you make to the form class that isn't show up?
I have seen this behavior in VS 2003 when changing web forms. If you change the class a web form inherits from (from System.Web.UI.Page to a custom class) VS chokes up and doesn't know how to "design" your page class.
I've seen where after you create a form and have it open in the designer things behave. Then you change the class and things act a little funny, but the designer is still present. However, then you close the design view of a form, then open it up again, VS can't figure out how to load the designer for the form, so the design view of the form is not available.
It's possible you are experiencing this problem with windows forms. I wouldn't be surprised.
If you have changed the class that your form is derived from, you can try this workaround:
1. Open the form code.
2. Change the derived class back to the default (System.Windows.Forms.Form).
3. Close the form code.
4. Open the form normally (into the designer view).
5. Open form code again.
6. Change the derived class back to your class.
This is annoying, granted, but it seems to be the only way to work around the designer. I've used this process with web forms, hopefully it will work for windows forms.