That means the file has moved or been deleted from the disk. This does not happen by itself and certainly not just after making adjustments to it in the LR Develop module.
So please give the exact steps you did with this file before it went missing.
LR is just a Database the makes a record of, References, where the images are stored on your hard drives. So doing any edits to any image in LR does not move, copy or change the original image in any way.
I edited the original image in Develop, saved, exported to my external hard drive. Now I cannot re-edited because I am getting the "this file cannot be found". How do I work around this so I can re-edite the image?
The message means the file has been moved or renamed or deleted and Lightroom cannot find it.
You need to find this file in your operating system so you know exactly where it is, then reconnect it in Lightroom by clicking on the exclamation point icon, as described in this document: Adobe Lightroom - Find moved or missing files and folders