Booting powers off partway through, Macintosh HD stuck unmounted.

Computer details: late 2012 iMac, i7 core, 24 gigs of ram, 3 tb fusion drive. Running 10.11.6. I'm unable to boot at the moment, so all of that info is from memory and may be incorrect.

My computer got stuck on the beach ball, while running standard applications (chrome, discord, steam, nothing new). I went downstairs to have dinner hoping it would work through what was wrong and I could do a standard reboot. When I came back 30 mins later, the beach ball was still spinning, so I did a power button reset. When I tried to boot back into the machine, the apple logo appeared, followed by the booting bar. When the bar was about 75% complete, the computer powered down. This happens whenever I try to boot.

I've tried going into disk utility mode and repairing all of my drives. Disk utility found no problems during the repair, although I noticed that my Macintosh HD was unmounted. I tried booting into Windows via boot camp, that works perfectly fine. I've tried partitioning and reformatting the disk, which all failed to dismount the already unmounted disk. I tried to do a fresh install, and to boot from the startup application, both of which could see my external hd and my boot camp partition, but not my Macintosh hd. I've tried mounting the unmounted drive and nothing happens. I've tried resetting my nvram, which didn't seem to help. I've tried booting into single user mode and running fsck, which found no problems. I tried fsck_hfs, on disk 0s2, (Google suggested this one) which gave the error resource busy journal_replay returned 16. I tried fsck_hfs on disk3, which disk utility tells me is my Macintosh hd, and that gave the error hfs_ValidateHFSPlusVolumeHeader : Unknown volume signature : 2873. I've tried to forcibly mount the disk using mount -uw / and that gives me error = -1, permission denied. I'm at a bit of a loss as to what to do next.

As far as disk data goes, I've got a backup from an hour before the crash; I'd prefer to not have to use it, but it's not a huge deal to wipe the primary drive. I just need to be able to boot again.