This could be two things.
The video could be using a codec that iPhoto doesn't recognise. Unfortunately Apple don't seem to publish a list of video codec that they support. Even Apple's listing of image file formats is vague.
The video could be using a recognised format but the file could be corrupt.
I would try opening the file in something that supports a wide range of video formats, like VLC. If the file doesn't open at all, there is a good chance it is corrupt. If it does open, you can check the file format with CMD + I.