ASP.NET clear label after clicking on submit button

    protected void ExportToExcel(DataTable dt)
    {
        this.lblMsg.Text = "";   NOT WORKING

        string sFileName = "MyFile_" + DateTime.Now.Year + DateTime.Now.Month + DateTime.Now.Day + ".xls";

        GridView gv = new GridView();
        gv.AllowPaging = false;
        gv.DataSource = dt;
        gv.DataBind();

        Response.ClearContent();
        Response.Buffer = true;
        Response.AddHeader("content-disposition", string.Format("attachment; filename={0}", sFileName));
        Response.ContentType = "application/vnd.ms-excel";
        //Response.ContentType = "application/vnd.openxmlformats";
        StringWriter sw = new StringWriter();
        HtmlTextWriter htw = new HtmlTextWriter(sw);
        //Change the Header Row Foreground to white color
        gv.HeaderRow.Style.Add("color", "#FFFFFF");
        //Applying stlye to gridview header cells
        for (int i = 0; i < gv.HeaderRow.Cells.Count; i++)
        {
            //Change to Blue
            gv.HeaderRow.Cells[i].Style.Add("background-color", "#507CD1");
        }
        int j = 1;
        //This loop is used to apply stlye to cells based on particular row
        foreach (GridViewRow gvrow in gv.Rows)
        {
            gvrow.BackColor = Color.White;
            if (j <= gv.Rows.Count)
            {
                if (j % 2 != 0)
                {
                    for (int k = 0; k < gvrow.Cells.Count; k++)
                    {
                        gvrow.Cells[k].Style.Add("background-color", "#EFF3FB");
                    }
                }
            }
            j++;
        }
        gv.RenderControl(htw);
        Response.Write(sw.ToString());
        Response.End();
        Response.Flush();
    }

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<asp:Content ID="Content2" ContentPlaceHolderID="cphBody" Runat="Server">

    <script type="text/javascript">
        function ClearTextbox() {
            document.getElementById('<%=lblMsg.ClientID%>').value = '';
            return false;
        }
    </script>

<asp:Label ID="lblMsg" runat="server" Text=""></asp:Label>

<asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="btnSubmit_Click"
                         OnClientClick="return ClearTextbox();" />   

</asp:Content>

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