Step-1: Add a button and a label in .aspx page.

<asp:Button runat="server" ID="Button1" Text="Check Internet Connection" onclick="Button1_lick"/>
<asp:Label runat="server" ID="Label1" />

Step-2: Now on the click event of the button write down the code in .aspx.cs page. Add a namespace

using System.Net;

protected void Button1_Click(object sender, EventArgs e)
{
 try
 {
 WebClient wc = new WebClient();
 byte[] dataSize = null;
 dataSize = wc.DownloadData("http://www.google.com");
 if (dataSize!= null && dataSize.Length > 0)
 Label1.Text = " Connection is Available.";
 else
 Label1.Text = " Connection is not Available.";
 }
 catch (Exception ex)
 {
 }
}