Many possible reasons - an infection in early pregnancy that showed no outward symptoms, genetic problem with the foetus that caused it to be reabsorbed, poor insemination technique, enzootic abortion ..... If she's exceptionally good it might be worth running her through another tupping season (wouldn't put her with the tup now, unless you want to lamb in June - probably too late anyway, unless she's a Poll Dorset) but otherwise I'd cull.