You have to locate the photos/folder in your operating system (Windows Explorer or Mac Finder) and then you will know the information to re-connect it in Lightroom.
Windows by default moves directories if they are dropped at some place on the same disk. Otherwise, it would have been a copy operation and that takes time and the original files stay in place. I know by experience, that it is very easy to drop directories inadvertently into a neighbouring directory. I suppose it is the same for OSX, but I have no confirmation of that. So chances are great, that the missing files are on the same disk.
If you have accidentally relocated a folder, you need to put it back at it's location. Lightroom shows the original location, so keep that location in mind.
Open Explorer or Finder and use the OS search function to search the folder or a file in that folder (if you do not remember the exact name then Lightroom again gives you the name! Having located the original data at their new place you can easily move them back at their original place. After that, Lightroom will not show any more the missing files icon, as all happens outside of Lightroom.
thanks everyone for your replies. I had already tried finding the folder and individual file names in it using Mac 0S X but nothing showed up. I manually searched all the other folders in Lightroom and my external hard drive but nothing showed up there.
Luckily I still had the photos on my SD card, however when I re-imported them , they were greyed out as I had the don't import duplicates box checked. I re-imported them anyway as the original files were still showing as missing with a question mark. I still have no idea where the original folder is , I certainly did not delete it.
Thanks for your suggestions anyway
If you want to reimport, then you first need to "delete" the files in Lightroom. But as you have edited them, you will loose those edits.
A workaround will be creating a new catalogue just for those files, import (import will work, as no files are loaded in the database), edit the files (again) and get them out to the customer.
But one thing that needs absolutely to be clarified: You did your search outside of Lightroom?
One thing I still can imagine is, that you deleted the pictures (using Finder) or that you moved them to a second external disk, that is NOW no more attached to your system. Also: did you search the right file names? You can modify the file name during import.
Instead of importing into a new catalogue:
Ignore duplicates when importing
Lightroom determines a photo is a duplicate of another file in the catalog if it has the same, original filename; the same Exif capture date and time; and the same file size. You can instruct Lightroom to disregard duplicate files when importing.
In the File Handling panel on the right side of the import window, select Don’t Import Suspected Duplicates
Switch "Don’t Import Suspected Duplicates" off.