If the edge markings are there, and apparently as they should be - the chemistry and time/temperature should have been somewhere near.
That leaves one major avenue (at the moment) that I can think of - *massive* under exposure.
You didn't change lenses and forget to reset the f/stop and shutter speed ...? I haven't done that myself .. not more than two or three hundred times or so...
Anyway... what were the circumstances surrounding the exposure of the film? Lighting? - Studio flash? ... and how did you determine exposure?
-Or ... maybe some sort of major shutter failure?