It would seem that somehow (a reason that I would not know) that image or images must have been removed from the catalog. That would be the only explanation I can think of.
File Cannot be Found is a message you see if the image is not accessible in the location where it was originally imported into LR.
Usually this is because the image file has been moved with the OS rather than LR, or the device where the images are stored has a new name (or on windows a new drive letter), or the device where the images reside has been unplugged. It's also possible that the images are somehow invisible to LR due to security settings Has anything like this occurred?
As far as the edits being lost, if the image is in LR enough to say the original cannot be found, then the adjustments are still there, but how you're resolving the situation is removing those edits and you're reimporting.
LR does have a mechanism for hooking the image back up with the settings stored in LR, you right-click and choose Find Missing Folder/Image or maybe the wording is Locate Missing Folder/Image, but the meaning is the same, you are telling LR that the images are now in a different place from when LR imported them, and assuming the images are named the same and the folder structure is the same, but only the top-level folder or device-name is different, LR will remap all the image settings to the new images' location.
It is important to understand what has happened to cause the images to go missing, because usually you want to do the Find Missing image operation at the highest folder level where things are different. Look at the Folder structure in Library, do the folders, themselves have question-marks on them, or is it only the images that have the Missing indication on them? If it's whole folders of images with the question mark then you can Find Missing on those folders and the images in the folders will come back--assuming you didn't move the individual images around also. If the folders don't show as missing, but the images do you'll need to Find Missing on the images, individually, which will be much more tedious. If the device is what shows as missing then just plug-in the device or if the device has been renamed, then do the Find Missing to each of the highest--level folders on the device.
If images are missing individually because you've been moving them around into different folders OUTSIDE of LR, then stop doing that and use LR to move things around, that way the settings in LR will be moved at the same times as the images.
All this is kindof scatter-shot advice because you haven't said what has caused the images to go missing, if they were moved, or the folders were moved or the device was renamed or what else you think might have happened that caused the images to still exist but in different locations than when originally imported.
when I open a picture in Lightroom and click on 'Develop' it says it cannot find the original file.
In the grid, the file will have an exclamation point icon, and you can click on it to locate the original file. You do not have to re-import the photo, which will probably cause you to lose your edits. Details are here: Adobe Lightroom - Find moved or missing files and folders
The file has become missing because somehow it got remove or renamed outside of Lightroom.
Hello did you ever find an answer? I got FILE CANNOT BE FOUND incorrect error. I edited 100 photos, went to get cup of coffee and when I r returned 10 minutes later they were all missing! (LR classic cc)
Is there an exclamation point icon or a question mark icon on your photos when viewed in the Library Module grid?
Can you show us a screen capture of one of these photos as it appears in the Library Module grid?