Make sure the breaker for that circuit is turned off in the electrical panel.
Remove the wire nuts.
Separate all the wires by color. If your wiring is anywhere near code it should be 2 or 3 wires encased in plastic, a black wire, a white one, and a bare copper one. Make sure that the power is off once more (a
circuit tester is your friend — touch one probe to black, one to white to test the circuit) and twist the bare ends of all the black wires together and secure them tightly with a wire nut. Do the same thing with the white wires and the ground wires (sometime the ground wires will be screwed to the box itself if the box is metal.)
When you turn the breaker back on at the panel you should have a live line and working lights. If the breaker keeps tripping you have a short circuit somewhere and you should probably call an electrician.