Two months ago I seemed to successfully transition from LR5 on my old computer to LR 6 (Standalone) on my new computer.
Now hundreds of photos from 2001 to 2005 have only the preview available and I get this message:
“070416-IMG_1806.jpg” could not be used because the original file could not be found. Would you like to locate it?"
I'm pretty certain they were there when I first moved to LR6.
What could have happened to the originals?
It seems that the Files that were imported in Lightroom have been moved to another location on the computer.
So I would suggest you to please reconnect the images first, and then you would be able to develop them.
Check the article :Adobe Lightroom - Find moved or missing files and folders