Sorry to hear about this bad experience!
This sort of happened to me once, but it was more like I was holding poopbag/umbrella/leash so I dropped the leash. He didn't run anywhere but I got scared enough to think of preventative measures. From then on, I put a karabiner at the look of my leash and I hook it onto my pants/jacket/purse when we walk. It's very handy, especially when you run out of hands or just want to be hands-free during your walks.
Do you think he became afraid because you yelled at him for running away?
Just an FYI, in any case or situation,
even if he's a bad boy...make sure that you always always always say "good boy" when he comes to you. And try very hard not to feel angry or sound angry when he comes to you. Even if you don't say anything, he still knows you're mad. They're very intuitive!
I agree with
Magnus, I'd start all over again and this time with treats. (i.e. each few step he takes, give him a treat; and then many steps treat; and then end of each block treat...until he's walking normal again)
And also, go out for your potty breaks for a max of 10 min. When he doesn't potty, go back inside and wait 10 min. And then go back outside again and give the potty command. Keep repeating until he finally potties outside. Just make sure you watch him like a hawk during the 10 minutes indoors so that he doesn't relieve himself inside. Eventually he has to potty outside. When he does, celebrate and make sure he knows he's a super duper very good boy!! To this day, I still tell Scottie to "go poopoo" and "GOOD BOY! Good Poop!" (or pee) when he goes. And I don't care what people on the street think!
It may be annoying to go in and out every 10 min until he potties...but this is better than standing outside for x amount of time (according to most trainers).