Maybe a coincidence.
One of my Macs ceased to boot after I toyed with Sierra – flashing question mark, Startup Manager found nothing more than an 'EFI …' option and selecting that one and only option did not cause Recovery OS to start. A reset of NVRAM did not work around the problem.
Happily, the Recovery OS slice was good enough to boot a more modern MacBookPro8,2 with the apparently troublesome older Mac connected via FireWire in target disk mode.
Eventually (I can't recall the details) I found the local Recovery OS (not Sierra Recovery OS) working as expected, so I erased the OS X startup volume and used Recovery OS Time Machine to restore a released version of the OS from a FreeNAS box.
After booting the restored OS, with support for ZFS, use of 'zpool status' found a growing number of errors in a pool that's encrypted with Core Storage. I began a scrub, but soon decided to shutdown the Mac and use something other than OS X to check the drive (SSHD).
I booted from Ultimate Boot CD (UBCD) and used HDAT2 to perform a 'powerful' READ/WRITE/READ/COMPARE test. That test began a few hours ago, it's around sixty-two percent complete and has found no error. I'll review this later …
… in the meantime, I'm reasonably certain that my case was pure coincidence – a disk beginning to fail, in a way that's not detected by the OS, whilst I toyed with Sierra. YMMV.