Step into the debugger:

$mail->SMTPDebug = 2; // Set to 2 to see detailed debugging output
$mail->Debugoutput = 'html'; // Change this to 'error_log' to send output to the server's error log

Hopefully the output gives further clarifications.

Another guess would be that your website has reached some monthly quota of google services.

Also check your server logs for any weird occurrences.

Enabling TLS encryption, like @life888888 pointed out, is something that depends on the implementation. If you insist on using SMTP, you have to enable TLS. This post goes into further details. Here is @life888888's advice. I repeat it here for better visibility:

$mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS;

If nothing helps, contact the google support.