First of all, thanks for being a hero!
If you haven't yet, search of the bike from the search page. Ideally, you'll have a 529 Shield or the serial number of the bike. Bike's serial numbers are usually on the bottom bracket (the area between the cranks). There may be multiple numbers there, so you may want to try to search on them separately.
If you don't get a hit, it may still be registered but not reported stolen yet. We recommend posting the bike on our Found Bike page, so that even if the owner didn't register it before it was stolen, they may be able to identify it. Just click on the Found Bikes link at the top of the page to add the information about the bike.
You can also send us mail at email@example.com and we can see if we can help you find it. Just be sure to include all the information you have about the bike, including the make, model, color, shield ID, serial number and any identifying stickers that might be on the bike.