Hey Tristan4837,
You should be able to recovery the video if you deleted it within the last 30 days. If you go to your Photos app and navigate to the Albums tab at the bottom, you should she an album that is called Recently Deleted. In there you should be able to select the video and return it to your Camera Roll.
Organize photos and videos
http://help.apple.com/iphone/8/#/iphf14943e
Recover a deleted photo or video. In the Recently Deleted album, tap the photo or video, tap Recover, then tap Recover Photo or Recover Video to move the item to the Camera Roll or, if you use iCloud Photo Library, the All Photos album.
If you do not see it then the other alternative would be seeing if you have a back up in iCloud or iTunes and to restore from that back up to get it back. If you go this route, you will be replacing the contents of your iPhone with what is on the backup. Any new information that is not in your back up would be lost. For how to restore from a back up take a look at the article below.
Restore your device from an iCloud or iTunes backup
https://support.apple.com/en-us/HT204184
Take care,
-Norm G.