Authorize.net is DOWN. Tendenci clients that use Authorize.net, which is quite a few, are being affected by an outage at Authorize.net.
As I type this at 12:10 PM on Friday the service is still down. Authorize.net is updating the status using twitter at: http://twitter.com/AuthorizeNet.
What can you do? They are working to restore power so most likely we just need to wait it out. However, you can set up a PayPal account as a contingency plan in case you want to switch merchant providers.