Generally if a chicken is frightened by a dog and had a heart attack she would not have survived, there is a chance however that she has suffered shock and in this case would be very quiet, often they fluff themselves up and are dull and lethargic. I would advise keeping her warm and quiet and closely watch her. Encourage her to eat and drink and if you are not happy get her seen by a vet
By Hannah James-Roll BVSc MRCVS (Professional Advisor)