If i supposedly returned a rental car back in less than perfect condition, will i have to pay anything?

I had an accident (not my fault), and i had a rental car for a week. When i returned it they said something under the car was missing WTF!!!, anyways i think they are trying to screw me over. it was like a really young stupid guy at the rental place. What will my insurance do? will i have to pay anything?please advise.