Troubleshooting Electrical Circuit Problems?

So tonight I was installing a better ceiling light in my kitchen. When I went to turn the lights on to test them they worked, but as I finished putting in the last screw, the lights in the kitchen, dining room, TV in the living room, and outside light on front porch and side porch all went out and won't come on. Other lights on the living room work, just not the TV. All outlets and appliances in my kitchen work. No circuit breakers tripped. I used my voltage tester and I'm getting power from the circuit breaker panel, but nothing to the switches. What could be wrong and how do I fix it?