You could have done many things wrong. To begin, check if the ground wires are connected properly. Next, check all your connections (WITH NO POWER!! ) with a multimeter. Enjoy.
Make sure you attached the black, white and ground wires firmly, have turned the breaker back on in the panel box, and the appliance your testing with is turned on.
First and foremost, did you turn the breaker back on? Then make sure that the wires are to the appropriate locations:
Black to Brass
White to Silver
Ground(uncoated) to Green
Check to make sure that you have the wires on the correct screws. You may have the hot and neutral reversed. Gold screw is for the hot "black" wire and the silver screw is for the "white" neutral. Green screw is for the ground. Also make sure that the breaker is not tripped.
