Videos with exclamation point inside a circle in bottoim right of thumbnail

Since I updated the last time, I have occasionally had videos that won't play. They have an exclamation point inside a circle in bottom right of the thumbnail. There is nothing in my iCloud storage; they have simply disappeared. How do I get them back?

Most recently, one disappeared when I was editing it because there was no room on the hard drive for the new version and the old. Instead of just leaving things as they were, the phone appears to have deleted the original video. How do I get it back?