car mechanic question

4 things could have happened:

1. in the process of connecting the spotlight, you shorted to ground.
2. the 55w spotlight is defective...breaking continuity in the circuit
3. you exceeded the current capability of the battery. (somewhat unlikely if it's a car battery). Perhaps this battery is very weak and needs charging?
4. blown fuse or faulty fuse holder. I'll assume you checked this.

5. Spotlight is off. Look for switch

The resistance of the 55w spotlight decreased the current flow so that

(E=I*R ) As you add loads to the series circuit you increase the resistance while the voltage remains at 12V. As the Resistance increases the current decreases. (E/R=I) As the resistance increases the current drops until you have no light. In a series circuit it does not matter that one load has a lower resistance than the others since all resistance adds in a series circuit. In a parallel circuit
the total resistance is always lower than than the lowest resistance load. In a series circuit the total resistance is the sum of all the loads resistance added together.

I understand Ohms law. As a different way of trying to figure it out I made this different circuit. A fully charged car 12v battery. I connected two 18w globes in series. They shared the voltage app 6 v each. (Kirchoffs law)
I then added a 2w globe. The 2w globe shines but the 18w globes go out. Obviously there is a circuit or there would be no light shining. The volt drop across the small globe is pretty much 12v. Across the 18w globe is a small amount of mVolts. I cant explain this.

must be a 5th explanation because it is none of these

The voltage drop across the small (2watt) lamp is somewhere close to .631 volts. The voltage drop across each of the (18w) lamps is somewhere close to 5.68 volts. What is the voltage rating of the lamps. All I can think of is maybe the addition of the (2watt) amp caused just enough additional voltage drop so as to not allow the (18w) amps to produce any light. The voltage at the (18W) lamps dropped from 6 volts to 5.68 with the addition of the (2w) lamp. If the (18W) lamps are 12V rated then maybe they cannot produce any visible light at 5.68 Volts. I don't know about your voltage readings as the drops across all the loads added should come to 12V. Are all the lamps 12V rated?