Yikes, that's horrendous. Good luck sorting it out. If you have a new heater in there, it may just be better to let it be and warm up slowly anyhow, I'm not sure what more you can do in the meantime. A quick temp swing back upwards may be just as bad overall as the temperature dip. If there is a silver lining, "too cold" usually is less deadly than "too hot."
Good luck.
Man I feel so terrible for you..