A whole lot of things can cause them to lose it. I would try again if you can get the stallion owner to honor.
I lost my first foal. My mare carried basically full term and aborted on day 336. Pretty filly (what I wanted). Nothing seemed wrong with the foal, we did a necropsy and they could find nothing wrong. Basically crap happens.
I did breed her again and got a very pretty and fancy filly in 2010. That filly was champion of her inspection and is now a lovely 2 year old. I’m waiting for my mare’s next foal due end of May.
It is hard to lose them…but just because you don’t know why she lost it this time doesn’t mean she will lose the next one. Sorry you are going through this!