Authorize.net has reported a resolution and are continuing to monitor this to ensure it is resolved.
See more here: https://status.authorize.net/
Posted May 26, 2015 - 14:35 PDT
We are receiving multiple calls reporting Authorize.net and credit card authorizing issues. Authorize.net has confirmed an outage on their Twitter account. They are posting updates on their status page as well.
You can read their postings here:
https://twitter.com/authorizenet https://status.authorize.net/ We apologize for the inconvenience. We will continue to monitor Authorize.net and continue to update this page.