My MacBook flashes the folder with question mark.

I have reset the nvram and it didn’t help. Before reinstalling the OS I want to backup some files on the drive, but I don’t see my main drive in diskutil. I only see the OS X base system partitions.

this happened right after I did a force reboot. My disk ran out of space as I was downloading a large set of files, had many apps open, and system was no longer responding or and was complaining about being out of space just as it slowed down as stopped responding.

thank you!