Call the fire department. Let them know the situation and find out how soon they could be there. Run down the steps, run into the MD and let them know the fire department has been called and you are on your way to reassure the cast and audience that the situation is being dealt with and please remain seated. Ushers will be around momentarily to usher you to the lobby.
After that, inform the crew/cast/ FOH what is going on. Get everyone possible a flashlight. A regional theatre may not have the biggest lobby, but it will be a more relaxing holding dock for the audience than the black of the theatre. If the actors and musicians (assuming live orchestrations and that a sound system is not needed) are able and the room in the lobby allows, inform the audience* of a rare opportunity, and start the show at an appropriate french scene in the lobby. (*Ask the Managing Director if they would like the honors of giving this "curtain speech" and thanking the audience for their patience, plugging the wonderful versatility of the production, etc.)
Of course, as the audience had been filing into the lobby, the crew and follow spot operators had been moving all practical set pieces and props into the lobby for such an event.
(would that be a good approach to take? any feedback would be great!)