I am not sure if I even had a 401k set up, but I cannot find any information on past business'/employer. I recently found two 401K plans that I was contributing to with companies that are still in business and were pretty easy to find as the older one gave me a phone number that directed me to the institution where it was sent to a temporary account. The other one is with Fidelity and much more recent and I was able to simply go to that website, also, I had an statement. The possible plans I am trying to research are much older- around 15 years ago.
It is a difficult matter to find information regarding a plan that was sponsored by a company that is no longer in business. However, if you did have a balance of more than $5000 when the plan terminated and they were not able to find you when they were to distribute all participant monies from the plan, the plan is responsible for sending your money to the state in which the company existed. This is referred to as having your money "escheat to the state." So if this did occur, and you contact your state offices, you may find that they have your money. If this is the case, you will then have to apply to the state to have them send you the money they have been holding. If your account was below $5000 and you can remember what investment company or pension administration company handled the investments and record keeping, you should contact them to find out if they can help you get information regarding what happened to your money.
Good luck with your investigation.