You have a short in one of four places, the wire bringing power to the switch, the switch, the wire bringing power from the switch to the light fixture or the light fixture.
Start by buying a cheap $3 electric tester, has two leads and a little bulb that lights up.
1 Take down the light fixture and take out the switch. Test the incoming hot wire to see if you are getting power. If not the problem is the wiring coming to the switch. If you are getting power to the switch it is the switch, the fixture or the wire from the switch to the fixture.
Next connect the hot incoming wire to the hot wire going to the fixture. Go to the fixture and text for power. If you have power it is the switch or the fixture. If not it is either the wire going to the fixture or the fixture it self.
You get the idea, keep taking things out of the loop and testing until you figure out what is wrong. Take down another fixture and wire it up to the fixuture in question to see if it is good or bad.