Tell her while you were walking home three ninjas attacked you. You were able to fend them off but not before they stole the ball. Get some eye drops and put them in your eyes before talking to her so she thinks you are crying, there is no way she will yell at you then!! OR you could tell the truth and pay for the ball. I personally like the first option myself :)
you should tell hr the truth.it's not nice to lie to her.the sooner you tell her that you lost it and make up for it the less likely she'll be mad at you for losing it.and if she has to pay for it you give her the money to pay for it.