Cash prize draw as an incentive to complete a survey

To encourage consumers to fill out a survey, we offered entry into a cash prize draw as an incentive if you complete the survey. 5 respondents will each win a certain amount of money. After capturing the respondents details, my systems crashed and I lost all the data, I tried everything I could and there is no way of retrieving that data. What do I do ? I have no way of knowing who completed the survey, so how do I choose five winners. Is this form of incentive permissible?