@Rachel851 If you are talking about a security deposit that was listed on your reservation, it was never actually charged to your account- it is only charged if a host reports that you've done damage. Check your bank account or paypal account and you'll see the security deposit wasn't charged.
If the host asked you for a security deposit in cash when you arrived, they are not supposed to do that, you should report that to Airbnb, and you'd have to deal directly with the host.