This can be just a bad switch. I would take off the switch plate, and verify that the switch itself is working.
Easiest is to use a voltage sensing probe. These are tremendously useful tools when you are working with electricity. With the power still on, CAREFULLY remove the outlet cover. Turn the switch off to the light. Now, one of the black lines running from the switch will be "hot", and the other black line will show no power. If you now turn on the switch, then both black lines from the switch will be hot.
If you cannot reach into the box with a voltage probe, then you will need to turn off the power into the switch box. Make sure that all circuits into the box are dead. Now, remove the switch. With a continuity checker (a multimeter is another great tool to have around the home, and not at all expensive) verify that when the switch is off, there should be essentially infinite resistance between the brass screws on the switch (where the black wires connected to.) Now turn on the switch, and that resistance should go down to zero.
If the problem is in the switch, just buy a new one to replace the old.
If the switch tests ok, then the problem is with the line to the fixture, or at the fixture itself. With the voltage sensing probe, I would now check that the line at the light fixture shows power. If not, then it is a problem in the line, and an electrician will be helpful.
REMEMBER: ALWAYS TAKE EXTREME CARE AROUND ELECTRICITY.