If a disk fails in a fault-tolerant array such as RAID 5, you just replace the disk and carry on.
A fault-tolerant array is just that - designed to provide fault tolerance.
However, if there is a controller failure or an operator error, you might end up with a set of disks lacking the array configuration.
The set of disks may be either complete (including all the member disks) or incomplete (missing a disk or several disks).
While not having a complete disk set slows down the possible recovery, losing one disk is not necessarily irreversible.
You can then send the disk set to the data recovery lab, or try to get the data off yourself using ReclaiMe Free
Whatever you do, if at all practical you should make disk image files (sector-by-sector copies) of the array member disks.
These image files act as an additional safety layer if one more member disk dies while you're busy recovering.
Remember to make images of separate member disks, not the entire array.
The most difficult part of RAID recovery is the destriping,
the process of converting the striped array to the contiguous set of sectors as it is on a regular hard drive.
ReclaiMe Free RAID Recovery does exactly that, giving you a choice of several output options, and at no cost.
You can even make it interact with certain third-party data recovery tools,
providing necessary preprocessing to make them capable of handling the RAID.