FAS and V-Series Storage Systems Discussions
First, you need to identify the reason which makes it offline. If it is just fisk failure, replace it with new disk.
Which version and mode of Data ONTAP you are using? "version" command show you version and mode of Data ONTAP.
In 7-mode, run"sysconfig -r" to check status of disks. "sysconfig -a", "storage show disk" and logs(messages and ems) is also needed.
In Cluster-mode, run each command above by "run -node <nodename> <command>".