If you are booting to an older system (El Capitan or older), then Disk Utility will NOT show an APFS format drive. It's not a supported format before Sierra, and won't be visible at all.
If you have upgraded to High Sierra beta, and you have a Recovery system upgraded as well, then the APFS drive should be visible when you are booted to the recovery system.
But, I don't think you can successfully convert a (spinning) HDD to APFS.
So, if you still have the spinning hard drive, you will likely need to erase that drive, then reinstall macOS, then get your other apps and files from your backup.
You DO have a current backup, correct?