Sorry guys, I missed this thread. It took us a while to figure out what was going on as well.
We use BigCommerce as our cart provider and they decided to move us to a new server in an effort to balance the load on their servers. Unfortunately they did so without any warning to us and tech support reps had no idea it was happening either. When we called they were baffled as well, but after escalating the issue they found out that it was because of the server move. By that time, the kinks were worked out and everyone had access again.