Motor warning light

Hello friends, I have a Focus 2.0 L / 2006 with Duratec motor and has just ignited a yellow light on the board, the normal motor start indicator, which should turn off as soon as it turns on. I checked the oil and I was very low, I added but the light is still on. The manual does not warn about possible failures. They help me, maybe you can solve without having to stop the car until Monday, just this weekend! Thank you!