The algae is probably what broke the CYA down...that tends to happen when there is a bad outbreak. For now, get the chlorine up to 12-15, and keep it there by retesting and adding more 2-3 times daily until the water goes to blue/cloudy. While you're doing this, keep the pump running 24/7 and brush the pool at least daily. When the water goes to blue/cloudy, continue to run the pump to get the dead algae filtered out (this may take several days), but watch the pressure on your filter and rinse cartridges as needed. After the algae is cleared up, then add your CYA.

Janet