Authorize.net is the largest credit card and e-check payment service on the web and has been down for a while, which means that a lot of other services went down to, because they can’t accept payments anymore.
Fortunately, Authorize.net is working on this situation and they also got a Twitter account, which you can follow at @AuthorizeNet, where you can stay updated with their work. According to them, the problem was caused by a fire at their Seattle datacenter, which got down their transaction platforms, website, and backups.
Don’t they have a redundant system to prevent service interruptions.