Assuming the low beams come on and they are the same lamp a ground fault is NOT on the list of suspects,, If they are different lamps, then it could be but the odds are very very slight.
more likely a bad hi/lo switch or one of the connectors.. If your switch is column mounted there is a connector alongside the collumn, (if it's floor mounted then it's on the switch) another at the firewall and possibly one more somewhere between the firewall and the lights.
There may also be relays.. If I were deisgning a vehicle I might well use relays to cut down the wire run.. (But I don't design cars) and if the high beam realay failed.. But I doubt such a relay exists since I've never seen them as standard, only as after market upgrades.
Home is where I park it!