I can think of 2 possiblities
- The photos are being accidentally imported into a different folder (not the 2017 folder that you want). Are the photos visible in Lightroom's grid if you click on All Photographs and then search for them by file name?
- This is a bug in Windows (you didn't say what operating system you use) that only shows a limited number of folders in a list (I believe the limit is around 1600).
Adding to DJ's list:
In Lightroom's Folders panel, make sure the tree shown there looks like it does using Finder/Explorer.
As needed, right-click on a folder, select "show parent".
Thanks very much, everyone. The "show parent" hint did it. I just went through the steps, and it is back. DJ, I should have stated my OS, which indeed is Windows.
Here's hoping it stays put.