Thanks for confirming. Most likely your SSD has died, although there is a small chance it could be a Logic Board failure in the circuitry supporting the SSD. From my own personal experience every Apple laptop which I've repaired which has lost the SSD has always been to due a defective SSD. Power problems can cause an SSD to suddenly fail. The easiest way to confirm the SSD failure is to have it examined by Apple or an Apple Authorized Service Provider. If Apple confirms the SSD failure, then you can purchase a third party SSD from OWC to bring your laptop back to life. Beside most likely being cheaper than the Apple SSD, the OWC SSD will also be larger and faster.
One thing you can try is Option Boot the laptop and let the laptop sit at the Apple boot picker menu for an hour. Then power off the laptop for a minute. Then repeat the cycle two more times. Sometimes this power cycle trick can "fix" an SSD and bring it back to life.
It doesn't hurt to try an SMC reset and PRAM reset (hold the PRAM reset for at least three chimes). Try reseating the SSD as suggested by @Neil.
If you don't have a backup, then you can try contacting a professional data recovery service such as Drive Savers or OnTrack both of whom are recommended by Apple and other OEMs. Both vendors also provide free estimates.
FYI, sometimes letting an SSD sit for a year or more without any power can sometimes bring the drive back to life, but most likely the data will be corrupt or gone, but it might allow you to re-purpose the laptop if you don't have it repaired now.
