Woke to dead tank

35% Off ! All WYSIWYG fish and corals
This morning go to feed the fish and all but one small clown was dead. My main pump quit and my heater is in my sump. Temp dropped from 26 to 23. I am wondering if that small of a drop in temp is what killed them or was it lack of oxygen or something? This may be a dumb question but I have never had a main pump stop pumping water over night. Any help would be appreciated. Had 2 mature clowns, one puffer, 2 blue tangs, 2 gobies. Crabs and corals are all coming back ok now that temp is up?