1) Check to see if you have power at the fixture. If you don't have a tester, just use a radio (or a lamp). With the radio in the on position, touch the black wire to one prong and the white wire to the other prong. If you have power, the radio will go on.
2) If the radio goes on, bad fixture.
3) If the radio does not go on, the problem is elsewhere.
4) Most obvious place to check is at the switch. Figure out what line is going to the fixture and which line should be from the power source. Disconnect the switch and test the power source wire with the radio.
5) If the power source wire is live, the problem is between the switch and the fixture. Good luck with that.
6) If the power source wire is dead, make sure you don't have a tripped breaker. No tripped breaker and you have a problem between the panel and the switch. Best solution is to abandon in place the existing wire and run a new wire from the panel or other power source (tap into existing junction box).