What does it sound like..driving car with no oil?

I had been driving my car for about 3 weeks with the oil light on. I figured it had oil in it so what was the big problem. Well, the car started making strange noises. I could not explain the noise when asked by the mechanic. I did have the oil changed then and was told there was nothing to change, that there was no oil in the car period. am told the noise is some upper engine noise and wouldn't even be worth looking into as whatever it is would go beyond the value of the car, a 1992 Plymouth Acclaim. What could have happened?