You won't see the Fusion Drive under Disk First Aid, You'll see the discreet drives (HDD & SSD) that combined make up the Fusion Drive set. If you can't mount the drives then you have a deeper problem!
At this point you'll need to use the command line interface of Core Storage following this guide: [http://www.pervasivecode.com/blog/2016/05/23/fixing-a-mac-fusion-drive-that-disk-utility-cant-erase/|Fixing a Mac Fusion Drive that Disk Utility can’t erase]