Go though the harness and try and find a 12v switched wire. You may need to pull the radio to get to its wiring. Also make sure while there that you have a good ground.
Catoctin's right, you'll need to make sure that the wiring is connected and hasn't come loose. Assuming this is an aftermarket radio, just make sure that the positive (usually the red wire), and the ground (usually the black wire) are connected.
Most aftermarket radios will have a fuse on the positive (red) wire. It should look like one of these: