Hey Kerstine82,
We'd be happy to help out. Sounds like you don't have your Mac in your possession, as it was stolen. If that's the case, you can locate the serial number using these steps:
If you don’t have your Mac or it doesn’t start up, use one of these solutions instead:
* Find the serial number printed on the underside of your Mac, near the regulatory markings. It’s also on the original packaging, next to a barcode label. You can then enter that serial number on the Check Coverage page or Tech Specs page to find your model.
* The original packaging might also show an Apple part number, such as MQD32xx/A (“xx” is a variable that differs by country or region). You can match the Apple part number to one in the list below to find your model.
This is outlined further here: Identify your MacBook Air model - Apple Support
For more help with what to do when your Mac is stolen or lost though, please review: If your Mac is lost or stolen - Apple Support
We hope this helps out.
Take care!