If there is a cab company listed on your receipt, please contact the cab company directly, so they can check with their driver. For your reference, the cab/medallion number will be listed on your Curb receipt. The cab number is also available in the "Ride History" section of the app and at gocurb.com.
If you took a cab in NYC, please contact the TLC at 3-1-1 as they manage all lost and found inquiries in NYC. They will need the medallion number, which is the cab number listed on your receipt and in the app.