Get a wiring diagram. You can get free wiring diagrams at BBBindustries.com. Or Google search. The diagram will show how and where the radio gets power, and which fuses the power goes through. Then get a test light and see if you have power at the fuse. If the fuse is blown, you'll need to figure out WHY it blew in the first place, and you'll have more investigating to do. If the fuse is good but you don't have power at the fuse, the problem is somewhere upstream. Refer to the wiring diagram, and poke the test light at various points in the circuit to find out where the power is missing.
If the fuse is good and you do have power there, I would pull the radio and see if you have power right at the connector that plugs into the radio. If you don't, there's a wiring problem. Once again, refer to wiring diagram and poke the test light at various points in the circuit to find out where the power is missing. Keep in mind radios typically have both a constant power source (so it doesn't lose its memory for the clock, radio presets and other settings) and a switched power wire (so it automatically turns on/off with the ignition key). Make sure power is good on both wires.
If you have power all the way to the connector at the radio, use the test light to check for a good ground at the radio connector. If you have power and ground at the radio, there's no reason it shouldn't work. Replace the radio in that case. If you don't have good ground, refer to wiring diagram and poke the test light at various points in the circuit to find out where the ground is missing.
^^You can use this procedure to diagnose about 90% of ALL electrical problems on a car.