Guide
Your MP4 won't open: what the error actually means
"Cannot play", "moov atom not found", the player that quits by itself.
Try VLC first
VLC is far more forgiving than other players: it opens files the system player refuses, and on some kinds of damage it offers to rebuild the index on the fly by itself. It's free and takes a minute: do this before anything else.
If VLC opens the video but it stutters or stops halfway, the file is readable but incomplete. If it won't open it at all, what's usually missing is the structure, not the images.
What the error is really telling you
An MP4 is made of blocks. One, the largest, holds the pictures and the sound. Another, small one, holds the index: where each frame starts, how long it lasts, how video and audio line up. When a program says "moov atom not found", it's saying exactly that: I found the video, I can't find the index.
That's why a four-gigabyte file can be unreadable because of a few tens of kilobytes. The images aren't damaged: the card catalogue is gone, not the documents.
First things first: is your file recoverable?
The check takes two seconds, it's free, and the video is never uploaded anywhere: the analysis runs inside your browser, on your own computer.
See if it's recoverableWhat not to do, under any circumstances
- Don't run the file through a video converter: without an index there's nothing to convert.
- Don't let a program overwrite the original while it "repairs": work on a copy.
- Don't change the extension hoping it will open: the content doesn't change.
- Don't buy the first program promising a miracle before working out what damage you actually have.
How to tell which case is yours
You look at how the file is built: which blocks are there, which are missing, whether the declared sizes match the real ones. That information sits in the first and last few kilobytes, and it reads in an instant.
The free check on this site does exactly that, inside your browser, without uploading the file anywhere: it tells you whether the index is missing, whether the file was truncated, whether the header is gone, or whether there's nothing left inside to recover.
Questions I get every time
The video opens but it's black, or there's only sound.
That's a different fault: the index is there, but the picture stream is damaged somewhere. The work happens at frame level and often most of the video comes back, skipping the unreadable blocks.
I downloaded the file from WeTransfer and it won't open.
Try downloading it again: an interrupted transfer leaves a file shorter than the original, under the same name. The free check tells you straight away, comparing the declared size with the real one.
Are there free tools to do this myself?
Yes: untrunc rebuilds the index from a reference file, and it's free. It runs from the command line and isn't the friendliest thing, but it works. The paid service exists for people who'd rather not sit down with it.
If you'd rather I handled it
You send me the file and the reference video, I send you a preview of the recovered one, and only then do you decide whether to pay. Twenty euros per video, thirty-five for up to five files from the same device.
You only pay if the video turns up.