php mail function has stopped working?

[g.c.simons36]

Is there a reason the php mail function has stopped working ie: mail($to,$subject,$message,$headers); - used to work until a couple of days ago?

<?php
$to = "example@example.com";
$subject = "example subject";
$message = "example message";
$headers = "From: user@user.com";

mail($to,$subject,$message,$headers);

?>