$mail->AddAddress($email ,"$row->email"); so I had to replace this line with
$mail->AddAddress('@gmail.com'); I guess this happens because it doesn't get the data




<?php
$servername = "";
$username = "";
$password = "";
$dbname = "";

// Create connection
$conn = new mysqli($servername, $username, $password, $dbname);
// Check connection
if ($conn->connect_error) {
    die("Connection failed: " . $conn->connect_error);
}

echo "Connected successfully";

 ob_start();
echo "Connected successfully";

require_once('class.phpmailer.php');
include("class.smtp.php"); // optional, gets called from within class.phpmailer.php if not already loaded


$mail             = new PHPMailer();



$mail->IsSMTP(); // telling the class to use SMTP
$mail->Host       = "smtp.gmail.com"; // SMTP server
$mail->SMTPDebug  = 1;                     // enables SMTP debug information (for testing)
                                           // 1 = errors and messages
                                           // 2 = messages only
$mail->SMTPAuth   = true;                  // enable SMTP authentication
$mail->SMTPSecure = "tls";                 // sets the prefix to the servier
$mail->Host       = "smtp.gmail.com";      // sets GMAIL as the SMTP server
$mail->Port       = 587;                   // set the SMTP port for the GMAIL server
$mail->Username   = "@gmail.com";  // GMAIL username
$mail->Password   = "";            // GMAIL password


include("config.php");


if(isset($_GET['emailaddress']))
{

    $emailaddress= mysqli_real_escape_string($conn, $_GET['emailaddress']);

echo $emailaddress;

    //$sql= "SELECT username,password FROM users WHERE email = $emailaddress";
    $query = mysqli_query ( $conn, "SELECT username,password FROM users WHERE email = $emailaddress" );
    //$query = mysqli_query ( $conn, $sql );

    $row = mysqli_fetch_object($query);
       $strMessage = "<table>
    <tr>
<td>User name : </td>
<td> $row->username </td>
<td>Password : </td>
<td> $row->password </td>
    </tr>
  </table>";
$mail->SetFrom('@gmail.com', 'FromName');

$email = mysqli_real_escape_string($conn, $_POST['email']);
$mail->AddAddress('@gmail.com');

//$mail->AddAddress($email ,"$row->email");
$mail->Subject = "Account Details";

$mail->AltBody = "To view the message, please use an HTML compatible email viewer!"; // optional, comment out and test

$mail->Body = $strMessage;

if($mail->Send())
        {
        echo "<script> alert('Mail succesfully sent to $email.'); </script>.";
        echo '<META HTTP-EQUIV="Refresh" Content="0; URL=mainpage.php">';
        exit;
        }
        else
        {
        echo "<div align=center style="color:#FF0000; font-family: 'Times New Roman', Times, serif; font-size: 26px;"> Cannot send mail. $mail->ErrorInfo. </div>";
        }
        }

ob_flush();
$conn->close();


?>