Please send an e-mail to: orders@ableton.com
Subject: "Lost my Ableton Live Lite serial number".
Body: The name of the Line 6 USB hardware device that your copy of Live Lite came bundled with.
Attachment: Proof of purchase of Line 6 USB hardware device.
Ableton will be more than happy to help you with this issue.
If you require phone support (in the US), please contact them at:
Tel: (646) 723-4572
Monday-Friday from 12:00pm to 4:00pm (Eastern Standard Time)