Hey there bichngoc183,
We see that you are having some issues with playing back a video that you edited in iMovie on your iPhone. We would love to assist you with resolving this.
First, try force closing the camera app and restarting your iPhone:
Force close an app: How to force an app to close on your iPhone, iPad, or iPod touch
Restart the iPhone: Restart your iPhone
Also, verify if your photos and videos are set to be backed up to iCloud: Set up and use iCloud Photos
When reviewing this article, check out the section titled "Save space on your device" for instructions on how to verify this information.
If the iCloud photos are enabled in settings, try logging into the Official iCloud website to see if the video has been backed up: Official iCloud Website
If you are able to view the video on the iCloud website then review this article on how to download the video back: Download iCloud photos and videos
Hope these resources help.
Take care!