My macbook air 2013 shut down suddenly

My 2013 macbook air shut down suddenly. This happened while I was away from the computer for a brief minute. When I came back it was turned off. I tried to start it back up again and the turning on chime sounded, the gray screen appeared and the loading bar started to fill but but when it's about 25% full the computer just shuts down again. I ran a hardware test and it found no problems. I tried resetting the SMC but did nothing to help. If I can't resolve this issue, all my hard work from this past school year will be lost (I'm a teacher).

If anyone has any experience with this and can shed some light on what the issue and how they resolved it, I would really appreciate it. Also, I was thinking it could have been a overheating or fan issue, although the hardware test did not find any. I was using the laptop for quite a long period of time but inside my air conditioned house.

This is a computer I am using from work but it's the second macbook air 2013 that I have lost important work due to issues like this. I need to find a way to at least get it to where I can back my work up as I was about to connect it to my external hard drive....of coarse this happens before I could!!!