My Rescue dog will not pee

Hi everyone, I adopted my rescue only three days ago and he has not peed the last 20 hours. I have called the shelter I adopted him from and am waiting on some advice, but I thought I should post here for some tips too. He is so incredibly frightened by everything which I was expecting, but the day we took him home and yesterday he was peeing fine. Yesterday a loud sound from my neighbour scared him in our backyard so bad that he hid under our deck for about an hour and now he won’t go near the backyard or even the door. I tried to take him out the front door so he could pee but he is not leash trained and almost ran into traffic then completely froze up so I had to carry him back inside. I am feeling completely overwhelmed as this is my first dog and I just don’t know how to help him. Any advice would be greatly