The switch, or there is a dead short on the low beam circuit somewhere between the switch and the bulb.

If it's a short the breaker should reset after having time to cool.

You could unplug a HL and jumper from the battery to each terminal to see that the bulbs light.

Or better yet, with the lights off, unplug the head lamps and check, with an ohm meter, between the terminals of the harness and ground. If you get continuity where there shouldn't be, it's a short, otherwise it's gotta be the switch or a bad breaker. Breakers do fail sometimes.