Hi there,
first thing obviously is try a new bulb, and have a general inspection of the bulb connections and wiring connectors (where possible),
if its the front then most common issue is the ground wire has broken somewhere in the harness (strange but true), you could run a new temporary ground wire from the battery to the bulb to confirm it's a ground wire issue ,
if it's the rear then most common issue is the bulb holder, either the bulb holder connector or the bulb contacts are dirty, corroded or burnt,
first thing obviously is try a new bulb, and have a general inspection of the bulb connections and wiring connectors (where possible),
if its the front then most common issue is the ground wire has broken somewhere in the harness (strange but true), you could run a new temporary ground wire from the battery to the bulb to confirm it's a ground wire issue ,
if it's the rear then most common issue is the bulb holder, either the bulb holder connector or the bulb contacts are dirty, corroded or burnt,
