Was the calf bloated???
If it was then it probably was clostridium. We have problems with that at the dairy i work at. The strongest, healthiest calves get it. In a matter of hours they can be deader than a door nail... and will have been perfectly fine before that. If she was bloated and her eyes were sunken in, it was probably clostridium. If that is the case, the next calf you can give a vaccine to to "help" protect her. But its not 100% effective. If you ever find one bloated like that just squirt 10 cc's of pennicillin down their throat and it takes the bloat off really quick. Unless the clostridium is too far advanced, and in that case she will be dead either way. It ruptures their insides...
Im soooo sorry!!!! hope you have better luck with the next one. Was she scouring???