If the video file is not exported correctly. Here are some options you can try:

First, try to fully restart the device. A lot of issues can be solved easily by simply restarting your device. If you are having problems, try this first.

1. Press and hold the Sleep/Wake button for a few seconds until a menu appears. 
2. Select Power Off or Shutdown.
3. Wait a minute until the device has shut down.
4. Start the device by pressing the Sleep/Wake button. 
5. Start the app again.

If this doesn't work.

Reset the app. Tap the more button and select Reset All Settings. Restart the device. Don't worry your projects will be ok.

If this doesn't work.

Try to change the frame rate and export the movie again.

If this doesn't work.

Backup the project and delete and reinstall the app. Don't worry all purchases will be restored automatically.