Can not open the designer. It gives error

[C# Programming]

Hi,
I am using visual c# 2010 express.
All of a sudden I get this error when I try to open the designer of the form.

The class name '?' is not a valid identifier for this ********.

Note that the solutin compiles successfully.
I have chedked for '?' but there is none.

Any thoughts please? thanks

Detailed error is below:

Instances of this error (1)

1. Hide Call Stack

at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomParser.CodeTypeDeclarationFromCodeClass (CodeClass vsClass)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomParser.Parse(TextReader codeStream)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.MergedCodeDomParser.System.CodeDom.Compiler.ICod eParser.Parse(TextReader stream)
at System.CodeDom.Compiler.CodeDomProvider.Parse(TextReader codeStream)
at Microsoft.VisualStudio.Shell.Design.Serialization.CodeDom.CodeDomDocDataAdapter.get_CompileUnit()
at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.PerformLoad(IDesignerSer ializationManager serializationManager)
at Microsoft.VisualStudio.Design.Serialization.CodeDom.VSCodeDomDesignerLoader.DeferredLoadHandler.Micr osoft.VisualStudio.TextManager.Interop.IVsTextBufferDataEvents.OnLoadCompleted(Int32 fReload)