(one's) heart stands still
One experiences a very strong emotions, especially excitement, nervousness, or fear, such that it feels as though one's heart stops beating for a moment. I swear, my heart stood still the moment my wife began walking down the aisle on our wedding day. My heart stands still whenever the principal calls me into her office, even if I know I haven't done anything wrong!
one's heart stands still
Fig. one's heart (figuratively) stops beating because of strong emotions. When I first saw you, my heart stood still. My heart will stand still until you answer.