CloudStack Production Problem
I've seen this situation before and, in my case, it was because Xen API service wasn't working anymore because of lack of disk space (logs filled the disk). I followed this post to fix the problem: http://www.techmediaconsulting.com/index.php/virtualization/citrix-xenserver-run-out-of-disk-space
If it's a production environment and you have Citrix support is better to have them involved though. In my case it was a lab environment.
Daniel





