If you have another one, you need to submit to your veterinary diagnostic laboratory for a diagnostic workup. If at all possible, get placenta(afterbirth), and a maternal blood sample, too - oftentimes, when we're dealing with an abortion problem, the placenta is damaged, and there are no abnormalities in the fetus. In most cases, definitive cause of abortion is not determinable - but the diagnosticians are able to 'rule out' the more common causes that you and your veterinarian can do much about - IBR, BVD, Leptospirosis, Neospora, Coxiella, etc.
Had two cows from my own fall-calving herd abort calves about 2 weeks ago; caught the first one of 'em before the buzzards got to it, performed a necropsy and diagnostic tests - diagnosed it as a Neospora abortion. Second one was about half-eaten and pretty rotten, but I have little doubt that it also was a Neospora-induced abortion. No more abortions in the past two weeks, but those two young cows will be taking a ride to the salebarn the next time they rotate back past the barnlot.
Decent discussion of Neospora abortion, from my friend Dr. Arnold, here:
http://afsdairy.ca.uky.edu/extension/re ... oracaninum
Looking for Neospora & lesions it causes in bovine abortion/stillbirth workups has been standard practice at our lab for over 20 years, but until the past couple of years, I could count on one hand the number of suspected or confirmed cases we'd diagnosed. But, over the past two years or so, we've been finding it much more frequently - and I don't think that we were 'missing' the diagnosis in the past.
If the problem is Neospora related, culling cows that abort is essential - and prevent dogs/coyotes/wolves from having the chance to consume aborted fetuses, placenta, or dead cows.
There is a serologic test to determine if cows are infected or not - but it's fairly expensive @ $25/head. I have to do some more research on the vaccine, but what I've read so far isn't too encouraging.