Photos are considered missing in Lightroom if they (or the folders that contain them) have been moved, renamed or deleted outside of Lightroom. (Hint: don't do that!)
Here are instructions on how to fix the problem: Adobe Lightroom - Find moved or missing files and folders
The catalogs don't store your images. Your images are in folders on your hard drive. If you have moved those images then Lightroom no longer knows where they are located. dj_paige has provided you with instructions on how to find the missing photos.