Five months ago I got a telephone call from Hertz rental control asking how much longer I was going to keep a car I had returned three weeks earlier!
When I said it was returned, they asked if it was damaged because it had not been rented since. They then asked me to fax them a signed note stating that the car was returned undamaged with a copy of my rental invoice.
It's best to call Hertz and ask what the problem is? - and ask what you need to do to solve it.
When you cancel a reservation, print off the screen as evidence and always keep the rental return receipt. Now and again, these things will happen.