Zoom H4N problem

[StephyPephy]

Used it to record audio this weekend for a 5D mkii shoot.

Transferred the files to my mac.

The main audio clip I need is 440Mb, but does not play, has a duration of 0, even on the H4N device itself.

How can it have a huge file size and yet there is no audio?

Any ideas?

I hate to say it, but I have had this problem on 3 separate occasions during 3 major shoots. I can say with absolute certainty that I never changed the batteries. I invested in a Tascam dr100 because I cannot risk reliability issues. If there is a known solution to the problem, then I will happily go back to using it, in the meantime, I lost very important audio.