Don't worry, it can be easily fixed with MP4Fix.
All you need is another valid video recorded on the same phone using the same app.
You can watch the repaired files for free!
Saving or sharing them requires small one-time purchase.
MP4Fix is designed strictly to repair camera recordings and it doesn’t support fixing or transcoding videos downloaded from the Internet.
MP4Fix repairs videos broken by camera crash, dead battery and lack of memory. The app doesn’t fix hardware problems. If your SD card is corrupt, replace it before running MP4Fix.
This app uses LGPL licensed JAAD decoder library.
Experimental support for H265 (HEVC) encoder.
Couple bugs fixed.
MP4Fix Video Repair Tool