From the AWS Service Health Dashboard:
6:18 PM PDT Earlier today we shared our high level ETA for a full recovery. At this point, all Availability Zones except one have been functioning normally for the past 5 hours. We have stabilized the remaining Availability Zone, but recovery is taking longer than we originally expected. We have been working hard to add the capacity that will enable us to safely re-mirror the stuck volumes. We expect to incrementally recover stuck volumes over the coming hours, but believe it will likely be several more hours until a significant number of volumes fully recover and customers are able to create new EBS-backed instances in the affected Availability Zone. We will be providing more information here as soon as we have it.
Here are a couple of things that customers can do in the short term to work around these problems. Customers having problems contacting EC2 instances or with instances stuck shutting down/stopping can launch a replacement instance without targeting a specific Availability Zone. If you have EBS volumes stuck detaching/attaching and have taken snapshots, you can create new volumes from snapshots in one of the other Availability Zones. Customers with instances and/or volumes that appear to be unavailable should not try to recover them by rebooting, stopping, or detaching, as these actions will not currently work on resources in the affected zone.