We were changing a switch in our house, running one light, when we lost power to several other outlets / light switches spanning 3 rooms! Any idea what the problem is and how to fix it? It did not trip a circuit.

We changed out a switch in our dining room and lost power in the bathroom and the bedroom. The switch was on the same wall as a gfci outlet in the bathroom and it had to be reset. Then everything was fine. I don't know if that's your problem or not, but it might be worth checking out.