Enterprise Rent-A-Car

I rented a car from an Enterprise car rental location in Pueblo Colorado on Friday November 20th at about 6:45 p.m until Sunday November 22, 2015 at 8:30 a.m. A few hours after returning the car I got an Email from the Enterprise that the car I rented from them has a 20" scratch on the rear bumper. I picked up this car in the evening and this particular location has a very dark parking lot. I did not see any scratches on the car when I picked it up and I did not scratch it when I had it. I returned the car early a.m. the office opened up at 9 a.m. but I was there at about 8:30 a.m. I asked the two girls working there if they could check me in but they said they were not open yet and everything would be ok. Just drop it off they said, which I did, only to get an email a few hours later that my credit card deposit was being held until the damage recovery dept. looked it over. What do I do? I didn't scratch the car? I've been a Enterprise customer for over 10 years with no problems at all like this. I've never even had an accident in my entire life of driving and that's over 30 years. What do I do?