Here I have created sample using your code,I am not getting issue like you.

HTML

<div>
    <asp:GridView ID="GridViewcmt" runat="server" AutoGenerateColumns="False" DataKeyNames="CommentId"
        OnRowEditing="OnRowEditing" OnRowUpdating="OnRowUpdating" EmptyDataText="No records has been added.">
        <Columns>
            <asp:TemplateField HeaderText="UserName" ItemStyle-Width="80">
                <ItemTemplate>
                    <asp:Label ID="lblName" runat="server" Text='<%# Eval("UserName") %>'></asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtName" runat="server" Text='<%# Eval("UserName") %>'></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Subject" ItemStyle-Width="100">
                <ItemTemplate>
                    <asp:Label ID="lblSub" runat="server" Text='<%# Eval("Subject") %>'></asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtSub" runat="server" Text='<%# Eval("Subject") %>'></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Comment" ItemStyle-Width="80">
                <ItemTemplate>
                    <asp:Label ID="lblCmt" runat="server" Text='<%# Eval("CommentOn") %>'></asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtCmt" runat="server" Text='<%# Eval("CommentOn") %>'></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:TemplateField HeaderText="Comment" ItemStyle-Width="80">
                <ItemTemplate>
                    <asp:Label ID="lbimg" runat="server" Text='<%# Eval("Imageid") %>'></asp:Label>
                </ItemTemplate>
                <EditItemTemplate>
                    <asp:TextBox ID="txtimg" runat="server" Text='<%# Eval("Imageid") %>'></asp:TextBox>
                </EditItemTemplate>
            </asp:TemplateField>
            <asp:CommandField ButtonType="Link" ShowEditButton="true" ShowDeleteButton="true"
                ItemStyle-Width="150" />
        </Columns>
    </asp:GridView>
</div>

C#

protected void Page_Load(object sender, EventArgs e)
{
    if (!this.IsPostBack)
    {
        this.BindGrid();
    }
}
protected void OnRowEditing(object sender, GridViewEditEventArgs e)
{
    GridViewcmt.Visible = true;
    GridViewcmt.EditIndex = e.NewEditIndex;
    this.BindGrid();
}

protected void BindGrid()
{
    GridViewcmt.Visible = true;
    DataTable dt = new DataTable();
    dt.Columns.AddRange(new DataColumn[5] { new DataColumn("UserName", typeof(string)),
                        new DataColumn("Subject", typeof(string)),
                        new DataColumn("CommentOn",typeof(DateTime)),
                        new DataColumn("Imageid",typeof(int)),
                        new DataColumn("CommentId",typeof(int))});
    dt.Rows.Add("David", "Morkel", "2015-05-05", 123, 1);
    dt.Rows.Add("Jhon", "Carter", "2015-06-06", 252, 2);
    GridViewcmt.DataSource = dt;
    GridViewcmt.DataBind();
}

protected void OnRowUpdating(object sender, GridViewUpdateEventArgs e)
{
    GridViewcmt.Visible = true;
    GridViewRow row = GridViewcmt.Rows[e.RowIndex];
    string cmtid = (GridViewcmt.DataKeys[e.RowIndex].Values[0]).ToString();
    string name = (row.FindControl("txtName") as TextBox).Text;
    string sub = (row.FindControl("txtSub") as TextBox).Text;
    string cmt = (row.FindControl("txtCmt") as TextBox).Text;
    string img = (row.FindControl("txtimg") as TextBox).Text;
    GridViewcmt.EditIndex = -1;
    this.BindGrid();
}

I hope this will help you out.