unable to see code by double clicking

Hi all, I Have create a project in Visual Studio 10 ASP.Net C#, which have many pages. its working fine but suddenly I am unable to go to code view via double clicking. I tried to double click on button but he didn’t show me code view, then I tried double clicking on text boxes but same result.
then I create a new project and in this project double click working fine as I double click on button  I reached on code view protected void Button1_Click(object sender, EventArgs e)   { . ….
I Open again old project but problem is still there .
Need you help
Regards,
Abdul Waheed