C# VS2008 "this document is opened by another project"

I have an application with several forms.
The application runs fine, with all forms, buttons, textboxes and combos running just as they should.
Major Problem: I lost the form designs. When I open a form file for design in the solution explorer, I get a new empty form (like a new design) , not the form I designed (the one running well in debug).
When I try right-click, open with, C-Sharp Form Editor (Default), I get the messge "this document is opened by another project", which is 100% incorrect.
The problem does not happen with the main form that I can open for design.
This problem could have been casued by careless renaming or moving files, which VS is extremely sensitive to.  
I tried the following:
1. Moved the folder to another computer and fresh run with another VS application - the same.
2. Re-named the files in question - the same.
I am desparate, as it looks like I will have to redo the whole graphics (Luckily I still have the algorithms!).  On the other hand, as it runs normally, the data may still be available somewhere.
Can anyone help? (I found a few (very few) comments about this on the net, but none seem to be a solution)
Sam .