How to fix an MP4 or MOV that won't play after the battery died
If your camera, drone or dashcam lost power while recording, the clip's video is usually still in the file. What's missing is the index, the part that tells a player where each frame is. Cameras write it at the very end, when you stop recording. Without it, the file won't open. With one good clip from the same camera, the index can usually be rebuilt.
Copy the file to your Mac and keep the original untouched. Work on copies. Don't delete the "broken" file: it looks useless, but the footage is almost always inside it.
Why it happens
An MP4 or MOV file is mostly picture and sound data, plus a small index. While recording, the camera writes the data first. Only when you press stop does it write the index. If the battery dies, the card is pulled or the camera crashes before then, you get a file full of footage that no player can find its way through. That's why the file is often the size you'd expect for the length you recorded.
Things to try first
- Let the camera try. Some cameras, including GoPro and some Sony models, try to repair an interrupted clip when they power back on. Copy the card to your Mac first, then put it back in the camera with a charged battery and see if it offers to restore the file.
- Try another player. VLC can sometimes play a clip with a damaged index, though usually not one whose index is missing entirely.
Rebuilding the index
To rebuild the index, a repair tool needs one clip that plays, recorded on the same camera with the same resolution and frame rate. It learns the camera's settings from that clip, then walks through the broken file frame by frame and writes a new index.
Free: untrunc
untrunc is a free, open-source tool that does exactly this. It needs a reference clip, runs from the command line, and on a Mac you build it yourself with Homebrew. If you're comfortable with Terminal, it's well worth trying.
Point and click: Shutterkit Repair
Shutterkit Repair is a Mac app for the same job. Drop in the broken clips and one that plays, and it matches them up and shows you a free preview of each repaired clip before you pay. It handles MP4 and MOV with H.264 or HEVC video, and never changes your originals. (We make it, so we're biased.)
About the sound
Uncompressed sound, which some cameras can record, comes back with the video. But most phones, drones and action cameras record compressed AAC sound, and without the index there's no reliable way to put it back together. In that case you'll get the picture without sound. A good tool will tell you so rather than give you garbled audio.
If the clip opens but stops early
Sometimes the index survived but the end of the video didn't, for example when the card was pulled during copying. Then the clip can be trimmed to what was actually saved, which a repair tool does without needing a reference clip.
Avoiding it next time
- Stop recording before switching off, and swap batteries before they're empty.
- Don't remove the card while the camera is still writing.
- Use a card fast enough for your video settings. A slow or fake card can make a camera stop mid-clip; Shutterkit Check can test its real speed.
Updated October 11, 2026.