iMovie Source Clip Missing

Made a number of really good videos, mixed with pictures on my iPad Air. Moved them to Theatre with no prob. Watched on a number of occasions. Was tidying pics and videos up later on to make space by deleting some/lots. Now when I go to my movies ‘source clip missing’. OMG. I am devisated. Assumed that once the movie had been made I could then delete the original videos. This is obviously NOT the case though? I still maybe have the gopro original videos saved on my external hard drive. Can I re-import them to my ipad and all will be OK?

PLEASE can someone advise me what to do? So many happy hours spent making these lovely videos!!