Are you 100% sure you have imported the photos to a hard disk? What folder and disk does Lightroom think the photos are in? I ask because we have read reports of people importing the photos while leaving the photos on the camera card; this would produce the symptoms you are describing.
This is something that LR doesn't do without you doing it and without a dialog window popping up asking you to either Delete From Disk or Remove from catalog. Even if you accidentally hit the Backspace key that same dialog box would come up asking you what you wanted to do, Delete From Disk or just remove from catalog and also has a Cancel button on it.
So no it has never happened to me in the 9 years I've used LR on multiple different computers including a Mac for a couple of years.
Thank you for your responses.
I imported the Raw files and had them saved on my photo hard drive through LR. Had exited and backed-up at least four times before this happened. Memory cards are not in the computer and I had backed up the files to another disk directly from the photo HD.
I would not expect LR to delete my photos without me asking it to, as that would be very rude; but it just blinked out and now says "No photos in selected Folder". And they are totally gone from LR and my disk.
Only possible glitch that might be involved is that I was listening to music on iTunes on the same computer (Windows 8.1/ i7 processor) as I was working. Has never been a problem before.
I will now export JEPGs as I work through the files so I can at least send them to my client.
How do you know the photos have been deleted from the disk, are the in the Recycle Bin?
LR doesn't delete photos without you asking it to, either by using the Delete or Remove function in LR, itself, or by your moving or deleting or changing the driver letter of the photos outside of LR and telling LR to Synchronize the folder that used to have the photos in it but doesn't anymore.
You may have opened an old catalog from before you imported the photos but that won't delete the photos from disk.
The message "No photos in selected Folder" could occur if the photos are all in subfolders and you've disabled "Show Photos in Subfolders" in the menu, but, again, this won't delete them from the disk and you'd be able to see the subfolders by expanding the parent folder you've selected.
Yes ,they were are in the Recycle Bin. Unfortunately, I haven't cleaned it out for a very long time, so the files are interspersed with many others with the same name so too time consuming to sort them out. I emptied it ,so should be able to go get them if/when it happens again. Not sure if the develop information would be applied to them, will have to experiment.
It may be impossible, but all the photo files in the folder I was working in were spontaneously deleted and placed in the Recycle Bin. Would still like to know why, so it doesn't happen again.
Perhaps you could provide a detailed description of what happened from start to finish - import to loss....
Exporting jpegs when finished is not going to solve your problem..... let's work to find out what happened.
If you haven't already, turn on Auto XMP Writing in LR Prefs. That way if the files get deleted you should be able to undelete the XMP sidecar as well as the image, so you'll at least have your current edits when you reimport.