Probably your CoreStorage Logical Volume got corrupted!
I recommend to backup your data to an external drive with Time Machine and rebuild the FusionDrive from scratch.
Time Machine backup
- To backup your main volume, boot to your main drive
- Attach an external drive. The drive has to be formatted as a HFS+ Journaled volume. Check this with Disk Utility. If necessary repartition your drive. All data on the drive will be destroyed after repartitioning/formatting it. The drive has to have the size of the volume to back up.