Please follow these instructions: Adobe Lightroom - Find moved or missing files and folders
By the way, if you imported the photos from the external drive (which is what it sounds like to me) this is completely the wrong thing to do and you ought to remove these imports from the external drive from Lightroom (but do not remove them from the hard disk).
Do you have any exclamation mark on the images in Lightroom?
If Yes please check the following link: Adobe Photoshop Lightroom Help | Locate missing photos
Have you moved the images inside Lightroom or outside Lightroom?
Are you sure that you are in the right catalog?
Managed to solve it by restoring to one of the backups and 'locating the missing files'