Q: iMac, 2012, 27″, won’t boot, not even in safe mode

My wife’s iMac, 2012, 27″, El Capitan, won’t boot, not even in safe mode. The problem started while installing Mac Office 365 (2016), though installing that on my machine worked fine.

The following two pictures are from Mac Safe Verbose mode. (The pictures are from consecutive boots, as the camera didn’t want to focus, but the Mac’s behaviour is consistent across attempted boots.)

http://www.jdawiseman.com/2016/20160218_230538_mac_wont_boot.jpg

http://www.jdawiseman.com/2016/20160218_230059_mac_wont_boot.jpg

Disk utilities reported an error; though fsck -fy didn’t.

Please, kind people of Apple Support Communities, what should I attempt next?

Thank you.