fusion drive won't mount
I restarted my iMac Friday night, as I always do before running the backup app. The Apple logo appeared, the white bar went halfway across, then it went black. Happened multiple times, after I unplugged all the devices, unplugged the Mac itself for a minute, tried safe boot, and zapped the PRAMs.
My computer guy said the drive locked itself. He can't unlock it because it won't mount. Any suggestions, as to how to unlock the thing? Or access it otherwise?
iMac 27″ 5K, macOS 10.13