Which version of Lightroom? Which operating system?
Unfortunately, this bug has been reported a number of times, it doesn't happen a lot, but it's a very serious bug. In fact, the one time I know that Adobe looked for the bug, they claimed it could not be reproduced. As far as I know, your photos are gone.
Documentation of the bug is here:
Another case is here:
Please add your voice to this bug report:
Operating system: Mac OS 10
Version: 10.10 
You should run the Creative Cloud desktop app and update LR to 2015.6.1.
But in the end it is a bug in LR. Sorry for the lose of the images but next time remember, Copy image to HDD or Import/Copy in LR to HDD and the next step is Back Up images to some other drive.
I normally don't move images around once they are imported into LR and if I copy images directly from a memory card to a drive/folder before they are imported. But I have done it a couple of times over the years. On one of those occasions, the last time I used LR for this, I lost some images. But I had backups so no big deal. But from then on if I need to move some images from one folder to another I use the OS file manager and then reconnect them in LR.