To clarify, the Fusion Drive is a single logical volume created across both the SSD and the HDD. You can create a Fusion Drive using any SSD and HDD pair. In other words, a Fusion Drive is implemented in software and the drives' formatting. It is not special hardware.
If either drive fails, then the volume will be unusable. The same is true for a RAID 0 setup, for instance.
The non-failed device will need reformatting to work as a single drive. If you replace the failed device, you can create a new Fusion partition with Disk Utility on the new setup.
Then you will need to restore your files from your backup.
This is not that much different from any other setup: if a device fails, then you need to replace it and restore from your backup. You should always be prepared for the possibility of any storage device failing by having duplicate redundant copies of your data.
