If the rack your instances are running in are totally offline then the ec2 api unfortunately can't talk to the dom0 and tell the instances to stop/start, so you

If the rack your instances are running in are totally offline then the ec2 api unfortunately can't talk to the dom0 and tell the instances to stop/start, so you get annoying "stuck instances", and really can't do anything until the rack is back online and able to respond to API calls unfortunately.