Cause of the Sudden Death of My Dog?

My 14 year old Chihuahua/terrier mix just died suddenly. He collapsed in our driveway. Thereis no sign of trauma or blood. His eyes looked like they were bleeding and foam was coming out of his mouth. He was diagnosed with a heart murmur 6 months ago. He also has a hernia on his bottom. The vet did not want to do surgery where it was located. What do you think happened to our dog? I'm so saddened. I know he was old, but he was okay 30 minutes before we found him.