Home Lighting Mystery: any electricians on this board?

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Today one of the light fixtures in our home went out. It is a 3-bulb ceiling fixture, all the bulbs went out.

I replaced the bulbs with new bulbs, they don't work. I made sure the new bulbs worked in another fixture. I tried both the screw-in cf bulbs and the old-fashioned 60 watt incandescent bulbs.

I checked the circuit breaker, everything looks normal. None of the other lights or electric outlets in the house are affected, the lights and electric outlet immediately adjacent to this fixture work normally.

The light fixture is switched by a two-switch system, ie. you can use the switch downstairs to turn it on or off, and another switch upstairs will turn it on or off.

I can only assume the somewhere (at one of the switches, or at the light fixture) a wire has come loose. As a precaution I took out all the light bulbs to prevent an electric fire in case there is electrical arcing at the failed connection.

My plan was to call an electrician tomorrow, does anyone have any ideas about this?