Grayed out thumbnails

So a summary

• Some images shown as missing, Dark Gray ! shown in Grid View boxes, and Filmstrip Preview boxes as to indicate a missing photo
• Some of the missing photos do display a library preview in the grid view and the filmstrip
• Some of the missing photos do not show a library preview in grid nor filmstrip
• In the provided screenshot, one of the images without a preview is selected. In this case the Navigation Panel, and the Histogram panel are blank. Histogram does state Photo is Missing.
• In the example mentioned above, while the Navigation and History panels are blank, at least one bit of metadata , keyword tags, has text.
• In the example above, the selected photo has a DNG file type
• Does not appear to be any filtering on.
• No folders involving the photos are showing ? mark
• At least one folder, but not involving the missing photos does have a ? mark
• One hard drive in the folders panel has a ? mark. This is apparently a different hard drive than that with/without the missing photos.
• No duplicate folders to indicate possible capitalization error
• In the folders panel, their are some strange entry's, Other Lightroom device.
• Author has attempted using Finder to find the photos, no joy.
• Per the Author, the parent folder is named Images. INQUIRY, what hard drive is that on?

Please comment on any errors or missing info in the above.

A few questions, and sorry to beat a dead horse. A few of these you probably did state as trying, I may have missed or misunderstood.

• If you right click on one of the missing photos that do have a preview shown, and click Show in Finder. Do you get the error message about the image not being found, if also, note and record the file name and where LrC thinks it was supposed to be. Then click on Locate. Does clicking on Locate provide a solution?
• Same question for one or more of the missing photos that do not have a preview.
• If locate fails. Use Finder to search on the noted file name(s) and if found, try again.
• Fix the possibly missing folder. Is it missing? or was it removed. A messy folders panel represents a messy database, and can lead to issues.

• And this apparent missing drive, that should be fixed.
• Inqury, this drive named PHOTOS, does it contain your folder named Images?

• And does anyone know what these are:

And to finish up this long wordy reply. And I did not see it above , in case of disaster:

• Where do you keep your backup of your catalog? Does that backup exist? Any reason to think it might not be healthy.
• If you have a backup, a good backup, consider an integrity check.
• Where do you keep your backup of your original photos? Do any of the truly missing folders, show up in this backup. Please please tell me you do have a backup.

Why do you think the files are corrupted? the normal indication of a corrupted file is either it is displayed improperly (but not greyed out), or there is a black circle with a white exclamation point inside.

Why do you think the files are gone forever? You simply need to restore the photos from your backups (using Time Machine or other backup method).

Please post a screen capture. In Library Module, with the grid view, perhaps a grayed out thumbnail selected, Histogram visible, Folders panel Visible.