Newsletter Subscribe in Asp.Net

Question

ASPX Code

<div class="single">
<div class="input-group">
<asp:TextBox ID="txtsubemail" runat="server" class="form-control" placeholder="Enter your email address…" ValidationGroup="email"></asp:TextBox>
<span class="input-group-btn">
<!--<button class="btn btn-theme" type="submit">Subscribe</button> -->
<asp:LinkButton ID="btnsubscribe" runat="server" class="btn btn-theme" OnClick="btnsubscribe_Click">Subscribe</asp:LinkButton>
</span>
<asp:RequiredFieldValidator ID="require1" runat="server" ForeColor="red" ControlToValidate="txtsubemail" ErrorMessage="Please Enter Email" ValidationGroup="email" Display="Dynamic"></asp:RequiredFieldValidator>
<asp:RegularExpressionValidator ID="RegularExpressionValidator2"
runat="server" ErrorMessage="Enter Valid Email ID"
ControlToValidate="txtsubemail" Display="Dynamic"
CssClass="requiredFieldValidateStyle"
ForeColor="red" EnabledEventValidation="false"
ValidationExpression="\w+([-+.']\w+)*@\w+([-.]\w+)*\.\w+([-.]\w+)*" ValidationGroup="email">
</asp:RegularExpressionValidator>
</div>
</div>

.CS File

protected void btnsubscribe_Click(object sender, EventArgs e)
{
DataTable ddtab = con.GetdataTable("select * from tbl_newsletter where emailid='" + txtsubemail.Text.ToString().Trim() + "'");
if (ddtab.Rows.Count > 0)
{
Page.ClientScript.RegisterStartupScript(typeof(Page), "msg", "<script>alert('You have already subscribed')</script>");
txtsubemail.Text = "";
}
else
{
int res = 0;
string query = "insert into tbl_newsletter (emailid,subscribe) values('" + txtsubemail.Text + "','yes')";
res = con.ExecuteQuery(query);
if (res > 0)
{
Page.ClientScript.RegisterStartupScript(typeof(Page), "msg", "<script>alert('You have subscribed successfully.')</script>");
txtsubemail.Text = "";
}
}
}

When I enter the email and submit the button then the output is not shown in the database so guys help me this problem.