PHP mail（）函数停止发送邮件

Since 3 months. mail() function works everywhere. But yersterday evening... function stops sending email... users can't register, log in, even contact support from our web...

function mail() is on.. but if i check

if(mail($to,$subject,$message, $headers))
    {
        echo "Test email send.";
    } 
    else 
    {
        echo "Failed to send.";
    }

i got 'Failed to send.'

Any idea what may occur? and how to display error?

because when i using that one:

ini_set('display_errors', 1);
error_reporting(E_ALL);

Have no errors. they are set to on(php.info)