ERROR: SQLSTATE[HY000] [2002] No connection could be made because the target machine actively refused it

If the WAMP icon is Orange then one of the services has not started.

In your case it looks like MySQL has not started as you are getting the message that indicates there is no server running and therefore listening for requests.

Look at the mysql log and if that tells you nothing look at the Windows event log, in the Windows -> Applications section. Error messages in there are pretty good at identifying the cause of MySQL failing to start.

Sometimes this is caused by a my.ini file from another install being picked up by WAMPServers MySQL, normally in the \windows or \windows\system32 folders. Do a search for 'my.ini' and 'my.cnf' and if you find one of these anywhere outside of the \wamp.... folder structure then delete it, or at least rename it so it wont be found. Then restart the MySQL service.

I had a similar issue when trying to migrate a Drupal website from one local host to another. From Mac running XAMMP to Windows running WAMP.

This was the error message I kept getting when trying to access the pages of the website.

PDOException: SQLSTATE[HY000] [2002] No connection could be made because the target machine actively refused it. 
in drupal_get_installed_schema_version() (line 155 of C:\wamp\www\chia\includes\install.inc).

In settings.php, I've changed everything correctly, database name, user and password.

$databases = array (
  'default' => 
  array (
    'default' => 
    array (
      'database' => 'mydatabasename',
      'username' => 'mydbusername',
      'password' => 'mydbpass',
      'host' => 'localhost',
      'port' => '8889',
      'driver' => 'mysql',
      'prefix' => '',
    ),
  ),
);

After a couple of hours of mindless google searching I've changed the port to a empty value:

'port' => '',

And after that the site loaded properly.

Possibly using different Port for MySQL than using in your Code

$conn = new PDO("mysql:host=".SERVER_NAME.";port=3307;dbname=".DB_NAME, DB_USER, DB_PASS);

Both ports should be same.