The headlights, and the headlights only - not the parking lights, are protected by a circuit breaker inside the headlight switch.
^^^^^^^^^^^^^THIS^^^^^^^^^^^^^^^^^^^^
Start at the headlights and work back.
Take the lamps out, and do a check to see if the bulbs are burned out. Ohmmeter, test lamp, hook them to a battery...whatever method works for you.
If they are burned out, then Dana has mentioned one likely cause. If not continue on the main investigation.
Then look over the connectors and follow the wires to the ground looking for damage in connections or insulation, as well as signs of heat.
Check the insulation on the feeds carefully. If it chafed through on the headlight buckets, that would be a reasonable cause for the breaker to open. I've had it happen.
Then onto the bulkhead connector - the shop manual for your year will have which cavity has the high and low beam wires. On the inside do the same back to the hi/lo switch.
