You might have renamed ,Moved the Source folder or have removed the source( External Harddrive)
Right click on the missing folder select "Find missing Folder"
Then simply point it to the Folder location and select yes
Learn here how to recover them
Thank you so much! That worked!
Wait it recognized the new folder name but now there are Exclamation points on some of them photos in this "new" folder?!
Click on the Exclamation sign"!" on the Thumbnail of the missing Picture and Locate it in Explorer/Finder
All other pictures in the same Folder will be automatically relinked
Once done you could simply check all missing pictures in the catalog
In the Library module, choose Library > Find All Missing Photos to display missing files