It's been less than two days. Call the driver — they're usually happy to return your belongings. The call button in your order history is active for 48 hours after the ride.
More than two days have passed or no agreement was reached with the driver: Describe your lost item and we'll help you get it back.
I lost the phone I requested the ride with. Contact us from the email linked to your account or via the app after restoring your number and logging in.