Hi You can Do it With Following Code :)
protected void Button1_Click(object sender, EventArgs e)
{
if (TextBox1.Text.Trim() != "" || TextBox2.Text.Trim() != "")
{
// do whatever you want to do
}
else
{
Page.RegisterClientScriptBlock("","<script>alert('Any One number is Needed!!')</script>");
}
}