Unfortunately ou most likely do not have a legal claim. When you drove into the overflow lot, it would have been obvious to you that it was a field; therefore, by choosing to drive into an park in an unpaid field, you could be deemed to have "assumed the risk"--or accepted the chance--of damage due to the field's condition. (After all, you could have declined to park there--either not attended the festival; or parked on nearby street and walked; parked further away and taken a bus or a cab; etc.) Assumption of risk generally acts as a bar to seeking compensation, because it represents your own fault, and parties may not collect for their own fault.