Surely someone checked the bulbs first?!  s

Bulb's could be out, there could be a short, the switch could be defective.
Could be many things.

Blown fuse.  Blown bulbs.  Bad relay.  Damage to the wiring harness.

The headlights are usually on a self resetting circuit breaker. If there is
a short they will reset but go out almost instantly. Naturally if the
circuit breaker resets enough it will probably eventually fail.