There is a forum for PayPal questions:  https://forums.oscommerce.com/forum/54-paypal/ There are several recent topics that say that PayPal made a change that requires an updated certificate.  If you made no coding changes and it just stopped working, that seems the most likely explanation.  The up-to-date version is at https://github.com/gburton/CE-Phoenix/blob/master/ext/modules/payment/paypal/paypal.com.crt If that is not it, it is barely possible that your host updated PHP in a way that broke your site.  But the certificate is both more likely and an easier fix, so I would check that before talking to your host.