First replace the lamp with a known good lamp listed on the fixture or ballast label.
If this does not correct the problem, then measure the input voltage to the fixture
and to the ballast to verify the availability of supply voltage at the ballast input.
If the supply voltage exists at the ballast input leads, check for OCV, using
the cautions in A2 above. If the OCV voltage is correct as listed on the ballast label,
the problem could be a defective capacitor or ignitor. Change one of these component
one at a time to correct the problem. Generally, if the ballast shows no signs of
overheating and the OCV measurement is correct the ballast is good and there has
to be another component failure or a wiring problem.