He was kind. He could have refuse you to enter into Thailand because of this. Planning to stay longer than thirty days is not allowed on a VISA exemption, you need a VISA for that.
go to the police and report it. Visit the immigrationoffice, they know the number of your departurecard from when you did the 24 hour report with TM30 on arrival, they can easily replace it with a new with new number and reference it to your old.