I've uploaded photos into lightroom, edited exported and save them from my external hard drive. When I re-import them into lightroom, they show up as soon as I remove the external hard drive to start working on them they show up as missing. Why is that? This never used to happen before but I had to reinstall lightroom and I'm not sure what to do. Please help.
When I re-import them into lightroom, they show up as soon as I remove the external hard drive to start working on them they show up as missing.
Do I understand you properly that you are removing the external HD and then you want to edit them? That will produce the missing indication in Lightroom. What to do to fix the problem? Plug the external HD in. Alternative: create smart previews before you unplug the external HD.
By the way, the workflow of editing, exporting, then re-importing is never a good workflow. Just leave the photos in Lightroom, don't re-import, just continue to edit the photos the next time you come back to Lightroom. Note: Lightroom does not work like Photoshop and similar programs where you have to Open the photo in the software each time you want to do something with it.
I don't understand why you are reimporting images. That isn't necessary. Your images are not "in" Lightroom. In your case, your images are apparently stored on an external hard drive. If you disconnect that hard drive then the path to the images is not valid, and that's why the images are listed as missing.