As previously noted, a necropsy examination would possibly have provided an answer.
6 days out...ruptured uterine artery unlikely; not unheard of, but not high on my list of possibilities.
Retained twin &/or metritis bad enough to kill the cow...I'd have anticipated you noticing her not feeling well, walking with tail up, discharge and bad odor...
Depending upon age/breed/milking ability...hypocalcemia(milk fever) would be a consideration...as would hypomagnesemia (grass/winter tetany) - I've been seeing some cattle deaths here at the diagnostic lab, due to grass tetany for the past month or so.
Have seen pneumonia due to a really hot strain of Mannheimia haemolytica kill cows with what appear to be no premonitory signs noted by the owner -'she looked fine last night, found dead this morning, at the hay feeder.'
Hope you have no more problems, but if another one kicks over... have your vet do a necropsy, or get the carcass to the nearest veterinary diagnostic laboratory ASAP.