fileupload is not working in asp.net

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C#

ASP.NET

File-Upload

hey guys i think there is something wrong with my visual studio 2013. my fileupload is not provide any filename ,every time it is giving a null value , previously it was running correctly and upload a file too but now it is not working

code/////

i have create fileupload in asp.net webpage and a button to test that whether fileupload get a value or not.

<asp:Button ID="Button4" runat="server" OnClick="Button4_Click" Text="Button" />
                            <asp:FileUpload ID="FileUpload3" runat="server" />

protected void Button4_Click(object sender, EventArgs e)
   {
       string fname = "", fn = "";
       fname = FileUpload3.FileName;

   }

in the above simple code variable 'fname' is getting a null value always from fileupload.
what is the problem ? is this a problem with my visual studio?