The Fusion drive is an amalgam of the SSD and HDD. If either piece gets removed or fails, you lose all the data. It might be possible to retrieve some of it, but it would be a job for drive recovery experts.
If you have a Fusion drive, it is possible to split it into the two separate pieces, resulting in a standard two-drive setup. However this is a destructive process, you need to reinstall the OS and all your data afterwards, as well as the recovery partition. On Macs that came with a pre-installed Fusion drive, you'll also run into issues where Disk Utility wants to "fix" the Fusion drive, so disk management needs to be done with the command line tools.
If you really want to split your Fusion drive, make sure you have a backup — this will delete your data; then boot into the Recovery partition (⌘R on boot) and open Terminal.
diskutil coreStorage list, and note the long string that appears after
Logical Volume Group — that's the UUID. Then run
diskutil coreStorage delete UUID, replacing
UUID with the actual string. You can then reinstall OS X on the SSD or HDD and restore from a backup.