You can right click/command click on that image in Lightroom and choose Show in Explorer/Finder and it will display the image on disk.
If you want to further edit the same image with Lightroom adjustments in Photoshop, you can select the image > right click/command click on it > choose Edit in Photoshop.
And you can also export that image onto your hard drive to save it somewhere. Refer this tutorial for export : How to export photos from Photoshop Lightroom
In case if it is showing that the image is missing in Lightroom, then you need to search for that image name on your hard drive so that you can relocate it in Lightroom in order to proceed with export.
Thanks Chetna, but no I can't. Every attempt gives me a variation on "original can not be found". The only thing I can think of is that the file hadn't finished loading somehow and now it is gone from the chip and the trash.
Thanks for your ideas though,
If the file has been deleted from the disk, then unfortunately, there is no way to recover it or to export it from Lightroom.