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
There are many laptop lost posts in this forum and if u go through them u may find ur answer. The answer is bit tricky. The way u can trace a laptop is through its in-built network adapter's mac address. This solution is near impossible to implement. There are so many cases where terrorists hacks into a home wifi networks to send mail after their work. To stop those kind of activities there is a solution at ISP level. ISP will have to go beyond the first level of connection with its client and into pvt network of its users and log every pc connected to the given network connection at anytime. This will be illegal as privacy concerns will be there unless govt pass some kind of law. Also many websites try to track their user even if u visit once. U will be surprised that even if u log in from a different network or even from a dif city, some of these sites remember u. This is another way. There are many ways like if u have a auto log in active in ur some of the online services say gmail. The maintains a last account activity log with ip address. chances that the thief may try to log into those and u can get the ip address the thief of may be someone known to him. U may give this ip address to cops and this will be legal method. there are many ways but only thing u wont get is co-operation so it is near impossible to implement.