Videos turned black immediately after filming

I’ve just read somebody else’s post about this issue and they weren’t able to fix it. I’ve just filmed a Videos and taking pictures and all of the photos are fine but the full videos that I filmed. I’ve just turned black immediately. I can’t view them on iCloud either but why is this happening it’s happened to me a few times and I don’t understand why because now I have to re-film what I’ve done which is really annoying. I’ve got plenty of storage on my phone so I don’t understand. I can see how many videos I’ve taken and the length of them, but they just are black on my phone and on the iCloud. It has a little triangle warning.