Internal SSD not visible

When I boot up my Macbook Air (circa Sept 2013, running Mountain Lion), I get a folder with a "?", indicating the SSD is not visible. This started when I tried installing an Office upgrade, and powered off the laptop midway because it appeared like it froze :(

I ran AHT by pressing D at startup. It passed with "No issues found", refernce code ADP000.

I installed OS X on an external USB Flash drive, it boots up OK, indicating that there is nothing else wrong with the laptop. However running diskutil list when I've booted from USB doesn't show the internal SSD.

I have some important files on the internal SSD. Looking for guidance regarding what else I can do.