I have experinced a simular problem with my vm's as well. It appears that although the datasotre is back online ESXi does not mount the datasotre and doing a rescan / refresh does not help. What I have done in the past is to login to the ESXi server and issue a mount command
esxcfg-volume -m datastore_name this should force the ESXi host mount the datastore and allow the vm's to be powered on.
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Gilbert
Technical Director
Metro Business Group, Inc.