Diagnosis
The bulb is burned out. The door switch that activates the light is likely fine if the oven still operates.
How to Fix It
Unplug the range. The light bulb cover is a glass dome held by a metal wire bracket at the rear of the oven ceiling. Push the wire bracket to the side to release it. The glass dome will lower. Use a cloth or oven mitt to unscrew the bulb. It is a standard 40-watt appliance bulb with an intermediate base (E17). Replace it with a high-temperature rated appliance bulb. Do not use a higher wattage bulb.
If That Didn't Work
If a new bulb doesn't light, the socket could be faulty. Test for power at the socket terminals with a multimeter when the door is open (requires careful live testing). The door switch could also be defective, but that's less common.
Heads Up
The glass dome and bulb can be hot if the oven was recently used. Let everything cool completely. The bulb must be an appliance bulb. A regular household bulb will overheat and shatter.