I'm not an expert, but have done a troubleshoot and replacement. Here. is what I learned....
Start out looking for the light's manual/instructions. Then read! You're looking for what a correct installation looks like, operating voltage, other details.
Do you have a multimeter? If so, verify power is getting to the light cable end when it should be lit. This tells you whether it's the controls or the light.
If the light, I'm so sad to say the next step is snorkel time. Remove the light looking for installation mistakes (e.g mine didn't have any slack cord in the niche, which it should have) and then for a burnt out bulb. If you find that in a seldom-used light the next thing to expect is a leak. The bulb is easy to fix, but the leak will be harder. Cracks usually mean replacement. You can fiddle with epoxy, etc, but how many cycles of trial and error are you up for? Rubber gaskets can also leak. They can often be replaced separately.
There are many YouTube videos on lamp and entire fixture replacement; fewer on other kinds of fixes.
If your fixture is LED, I can't help much. I think they are mostly repair-by-replacement. Also can't help with Hayward controls.
Safety: No matter what, make sure everything is off before removing the fixture. Even if it's 24 volts.
Caution: If it's a halogen fixture, it shouldn't be on when out of the pool for more than a few seconds. It needs water for cooling.