Elaine and Kramer blew the electric circuit attached to her neighbor's apartment using a paperclip in the outlet in the hall. However, later in the episode, when they go back to feed the cat, they use Kramer's slicer. But because they'd blown the circuit, they technically shouldn't have anywhere to plug it in.
When they try to blow the circuit the light goes out in the hallway, so the power must have been out for the whole building. Obviously, the power was restored before they used the slicer in the hallway.