Don't just give up on finding your old 401(k) - these are your assets and you don't want to just lose 3 years worth of savings.
First suggestion is to try and dig up any old statements the administrator of the plan sent you. They should send a statement to you at least quarterly. Have you looked through all your old financial records?
Second suggestion is call your old boss or any old colleagues and ask them what happened to the 401(k) plan.
Third - most plans are required to file an annual "Form 5500" with the U.S. Government. You can search these 5500's for the name of your former employer at free websites like
Welcome to freeErisa.com!. If you can find a Form 5500 on an old plan, it will have contact information.
The final suggestion is go online and search for abandoned property in the state you lived in at the time. If you have not been receiving statements from the 401(k) plan administrator they may now consider you lost, and by law they would have turned your assets over to the state. Every state has a program to try to reunite people with their lost assets. You can do a Google search using the name of your state and "unclaimed property" and that should take you to a web site with information on how to track it down.