The metadata is there, but the images are not.
Photos is storing the metadata and edits separately from the original image. when you add image files to Photos, it will store the original image file either inside the Photos Library or reference it in its original location. in addition to this, Photos creates an edited version. This is a description of the edits to be applied and the metadata tags. Photos uses the version to compute the rendered image from the original. Apparently you are still having the version, but Photos cannot access the original.
Are the photos in question referenced?
I would inspect the folder where the missing image files should be. Select a photo that is not missing, but has been imported at the same day as a photo that is now missing. Copy the filename from the Info panel.
Now search with FindAnyFile for this filename.
Download Find Any File here: The trial is free:
http://apps.tempel.org/FindAnyFile/index.html
FindAnyFile let's you find files inside library packages where Spotlight will not search.
This way you can look into the folder with the original Master image files. It is organized by the date the photos have been imported to Photos. If you can find the folder, where the photos should be, you can open Time Machine and go back in time to the time where the photos have still been there and try to restore them to a folder on your Desktop and reimport them.
Sometime the problem is, that the filenames have become corrupted. The files are still there, but the filenames cannot beread because of an incompatible filesystem , ethnic characters, or similar.