Well, my life is saved because of this thread. I got the same problem, MBP suddenly can not boot after I upgraded to High Sierra (after a success installation rebooting). Knowing that I did nothing but upgrade the OS, I would never guess that it could attack the hardware. So I brought it to the Apple official reparation place. It was confirmed that the SSD is dead, with all my data just lost (lucky enough, I had backed up my upmost important works with Google Drive). They told me this could happen if there's an undetected malfunction in the hardware all along, that upgrading OS might bring up that malfunctionality to the surface. Meanwhile my warranty is off this early year.Had the SSD replaced FOC under consumer law!
So I stumbled upon this thread, and read about the consumer protection law above (which is not existent in my country as far as I know). But this inspired me to try to call the Apple Support and ask if I could get something like that consumer protection law, because in my case it's not from the user's fault. Well who knew, Apple support has been very helpful, they connected me to the Apple Asia Pasific support team on free toll call. After some confirmation steps, the next day they gave me a free replacement for the SSD ( I had to bring back the unit to the reparation place, they checked my unit's serial number, and it is automatically said that it's covered for the SSD, still need to pay for the technician service though, but it's ok!). Of course, i've been honest about the whole situation to the Apple Support team, and I am glad that it worked. So I owe it to this thread, who knows my experience could help others too.