If you did not install some sort of tracking software you are probaly out of luck. I don't even know how you could lose something as large as a laptop, i'd have to assume it was stolen.
Contact the police, and provide the serial number and any other info if at all possible. If you lost it at a college campus or store obviously check their lost and found. You could also try and contact local pawn shops and computer repair places with the info, although they may not be willing to help. The reality is its probably gone for good. If you had any sensative info like credit card numbers, social security, etc stored on there watch your credit reports and bills. Windows passwords are worthless and easy to get past