Hi there and welcome to Just Answer!
The best way to find your 401k plan would be to contact your former employer's human resource department. If your employer is no longer available, then the second best search tool would be to see if you have any old benefits information from your previous company, or any statements from the institution that held your 401k. You must have at one time gotten at least yearly or quarterly statements from them. If the 401k plan was at a brokerage or bank that merged or was bought out, that can be traced. If you know anyone from that company that you can contact who may also have a 401k plan, you might want to try that.
If none of the previous suggestions yield results, then you will most likely need to conduct an unclaimed property search. If there has been enough time lapse between your activity with the plan and now, the account may be listed on a state or private unclaimed property list. Here is a link to unclaimed property lists by state, usually indexed by name:
Here is a link to a company which will search specifically for 401k plans. You will have to give some information including name, email, telephone and address. This service costs $18.
If you can find the name of the broker or bank who previously held the account and would like the current contact information, we can help you with that too.
Best of Luck in your Search