Plugged in an electric heater and it knocked out half of the house is power. The breaker did not trip and there is some sort of power going to the outlets just not enough to power anything. The main power line from the meter box has been changed recently due to this same problem but it happened again. What could be the problem?

Turn off breaker. Pull an outlet, any outlet on that circuit. See if it was wired using the "quick connect" holes in the back. If so, remove and put under the screws. Do this to all outlets and switches. Turn on breaker.