What makes you say that?
ISO400 means it's more (2 stops) more sensitive than ISO100 film. It also means if you are taking your photos at very VERY bright outside and you can't use faster shutter speed and/or smaller opening on your aperture, you will severely over-expose your negative.
It's still very unlikely you get completely BLACK frame though. It is my understanding that those Kodak and Fuji throw-away one time use cameras use something like ISO 200 film in it. You can shoot all day long at beach and still come back with usable images.
I really think something went wrong here, either with your film or equipment. (or processing at lab)