Question Files vanished, then reappear on reboot?

Mar 5, 2019
Like all too many people, I've found myself elevated to "Family IT manager" because I took a computer class one time in high school. I'm sorry in advance for any stupid mistakes or misunderstandings that I may have made.

Yesterday evening, I restarted my parent's computer in response to it running slow. However, when I logged back in, the desktop had reverted to the default windows image, and all links and folders had vanished (The program icons were still there, but sorted by name instead of scattered across the screen like they normally are). I went to "documents" in search of the missing files, but there was only a single folder, with a fairly randomized name (I know, I should have recorded the name, but I didn't think of it at the time).
Obviously, I was becoming increasingly concerned about the possibility of a virus attack or ransomware at this point. I rebooted the computer, and was surprised to find that everything had gone back to normal, with no sign of the incident. A virus scan detected nothing, and there haven't been any problems since then.

Mainly, I'd like to know what the heck happened, and if there's anything that I need to do in response.
Does anyone have any ideas or experiences similar to this?