Do you mean that the thumbnails appear in Lightroom but there is a question mark icon or an excalamation point icon on the photo?
If so, the solution is explained in this document:
Thanks for your assistance.
It is an excalamation point.
I have attached some screen shots and you will also see there are some
? marks in catalog titles.
The Folders panel shows lots of missing folders.
You need to "find missing" on the folders before you can "find missing" on the images.
See the link in the first response.
Click on Merge