Due to a fire at the Authorize.net data center, all their systems, including their payment gateway and corporate website are currently down.
If you are using Authorize.net as your payment gateway, we suggest that you switch your payment processing option to “Manual” for the time being. You will then have the order information to process when things are up and running again.
If you're on twitter, follow @AuthorizeNet to hear their updates.
We’ll let you know when we learn more!