My dog has just died.. I don't know what to do?

My dear German shepherd Ava has ped away this evening about ten minutes ago. I had let her out after dinner to do her business, like every other night. She had somehow gotten out of the yard and ended up being hit by a car (the person who was driving drove away - I can't believe someone could be so heartless). I didn't know it, so my neighbor came up and told me Ava had walked up to her house and lied down and was having trouble breathing. I was getting ready to take her to the nearest animal hospital (45 min away) when she stopped breathing. I am heartbroken. She would've been a year old in February. She was my first GSD and such a loving, kind dog. In a way, she was my best friend. I have never experienced a pet's death and I just don't know how to cope. I'm coming to the internet to find some coping techniques and comfort.