First thing is always..check the fuses if it's multiple lights out, check the bulb if
it's a single source out. Don't just look at the fuses, they can be bad but not show it.
Replace them each in turn, with a new one. See if that fixes it.

If not, it's time to whip out the multi-meter, or a 12 volt test light.

Try the fuses, first. If that doesn't fix it, let us know and we can explore further.

Ern