MacBook Pro lost SSD during macOS High Sierra update
You should know that updating to High Sierra normally means to a change in the used disk format. (Apple introduced APFS for macOS High Sierra which will be used instead of HFS+). If you now updated the system on your SSD the SSD will probably be formatted as APFS and therefore the older version of OSX won't be able to read it's content.

Did you apply any encryption on the SSD system? Because I had trouble with my system too when the update produced a unknown failure and I tried to access the system partition using an El Kapitan system. (I was forced to wipe all data and sync back my time machine backup)

Maybe try to access the data by booting in recovery mode (because the recovery system should now also be upgraded to High Sierra)