It can have one of two meanings, normally. The usual one is that the image is either off-line or otherwise missing and LR does not know where it is because it either does not have access to the image or it is missing.
The second way you can get this ? is when you delete an xmp side car that LR was tracking and no sees is not there with the image.
With either you usually will get a pertinent dialog to either locate the image or in the other case, regenerate a sidecar, tell LR to ignore that it is now missing.
Have you tried restarting LR or putting aside the Pref file to see if then a dialog will show?
Don