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 a 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: Email us from the email linked to your account or via the app after restoring your number and logging in.