User actions will cause Lightroom to report a photo is missing. Specifically, if you rename, move or delete a photo (or the folder that contains them) outside of Lightroom, you will find that the photo is now considered missing by Lightroom. The way to prevent this is to do all of your photo management inside of Lightroom, not outside of Lightroom (or better yet, develop a workflow that doesn't require you to move photos from here to there)
BUT I still don't know what triggered the loss in the 1st place. I realize I have photos all. over. the. place which made locating them all a big time sucker. How do I prevent this from happening again? How to I export without losing them? I never had a problem with exporting to a desktop file before, but now I have them in so many locations on my old internal hard drive and back up on my external hard drive, it takes hours to locate them! Can I save my catalog so that if this happens again, I can just reload the catalog? I'm so lost!
The whole point of Lightroom's Library Module is to give you tools to organize and find your photos. It sounds like you are not doing this. If you have to locate photos from "all over the place" then you need to begin using keywords and other metadata to organize your photos, so from now on, if you want to find photos of (for example) soccer games, you simply use the "soccer" keyword.
Regarding exporting, you export from Lightroom only when the photo is needed outside of Lightroom (web, e-mail, print, etc.) and then the export is temporary. You delete the exported photo(s) when it is no longer needed. Then you no longer have to keep track of your exports which are "all over the place" and your problem is simplified to keeping track (organizing) the originals.