You have to go back until you get to a clean, straight piece of pipe and start over. That may mean you have to cut all the fittings off.
Is it broke off completely or is it just cracked?
If it's just cracked you can clean up the area with sand paper, a file and anything else that works and then patch it is tight, clean it out and apply some ABS cement to it. Hold the crack closed until it dries. For extra strength you can cut a piece of pipe or part of a fitting and glue it over the repair.
If the crack is bigger,
After it's cleaned up, grind up some ABS material from a piece of pipe or a extra fitting and add the appropriate ABS glue to make a slurry. Working quickly, apply the slurry to the damaged area. You can add more after it sets up or just keep brushing more glue on.
These steps will make a very strong patch that will last.