I had the same problem a few weeks ago! I was using the computer fine and then suddenly I couldn’t access any file from the SSD, only getting IO errors. Rebooting did not fix the issue, the SSD wasn’t showing up in dmesg anymore, I had to power off and on to get it back. (this is using the SSD from the kit)
EDIT: 20 minutes after writing this message, it happened again! This time I noticed that the LED next to the SSD (D23) was flashing at a fixed rate (about 2 Hz) even after a soft reset (Circle+R). I had to power off and on a few times for it to come back.