Can the JPGs be accessed when LR is closed? Does “accessed” mean opened by other software while LR is running or even after you shut down LR and perhaps reboot? Is the issue intermittent with one or two JPGs at a time or all your JPGs at once?
Lightroom never touch your original images for editing, at the time of import Lightroom import the smart previews for the image into the catalog . whatever the edits you perform that is just a instruction you setting in Lightroom catalog file.
If you are not able to access the jpeg file in Lightroom catalog and on windows, it means the files have moved to another place, drive or could be deleted as well. If you can preview the missing jpeg in Lightroom catalog you can extract the previews as a jpeg file.
It was a Windows thingy. I rebooted my PC and then it said that it found trouble with my external disc. It offered to fix it which I gladly accepted and then the folder with the missing images appeared again in the explorer and then also in LR. This was Win10.
I had a similar problem with 4 photos some days before but thought I did something wrong.
But now I remember that there is one task where LR does touch the original files: By changing the file name! And that I did a lot because I manually sorted negative-scans which I then gave a new sequenced file number.
So my lesson learnt is: Back-up before renaming photos.
And may be Adobe should think about a less destructive way to hard-code a photo sequence. Something like an auto-sequence-number for a metadata entry and then one can sort by that.
Renaming doesn't modify your files, it simply changes a value in your directory structure.
However, if you choose to write metadata to XMP, then the metadata portion (but not the image portion) of your JPEGs, TIFFs, and DNGs will be changed. To my knowledge, this has never caused corruption in the absence of hardware problems. That said, backups are never a bad idea.
Directory damage seems to have happened: the directory structure was damaged as I could not open the folder from the Explorer.
I thought that XMP means that a sidefile is created: the *.xmp file, and so no data is written to the image file. Can you explain?
Sidecar files are written for proprietary raw files (CR2, NEF, ORF, ARF, etc), but image files that are in the public domain (TIFF, DNG, JPEG) carry their XMP data internally, and that's where LR writes them.