It's been about 20 years since I've had a 401k but do not have any paperwork. The company that I used to work for, is out of business. How can I track it down?

Do you know any of the old workers? Any chance of some old income tax statements?
Old records ?

They mail you quarterly statements... if you failed to update your address when you moved like you are supposed to... then this falls on you.

The 401K program was managed by a third party, not your old employer. However with them out of business... fat chance asking them who managed it for them. There are a LOT of different places that do it. Good luck guessing which one it might be.

When you leave a company with a vested retirement benefit, that company is supposed to report this to the SSA. So when you apply for Social Security benefits, they will inform you of any that you have. You can also check with pbgc.gov

As noted above, your account assets are with the plan administrator, not your old employer, so the goal here is to find out who that is. This is most likely a mutual fund company, or an insurance company, or a bank that your old employer contracted with to administer the plan for them. If you didn't keep any of the paperwork from when you originally enrolled, or statements that you received in the past, then I suggest start by contacting any colleagues from your old job or your old boss to ask who the administrator is - then you can call the admin and update your mailing address with them. If that doesn't work, you can try the National Registry of Unclaimed Retirement Benefits (NRURB). They are responsible with keeping over 50,000 individual retirement accounts for individuals who have forgotten 401(k), profit sharing and other company sponsored plan balances that have been abandoned: https://www.unclaimedretirementbenefits.com/

An additional suggestion is to go on-line 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. Here is a free web site that lets you search your state's database of abandoned assets: MissingMoney.com Unclaimed Property FREE SEARCH - Officially endorsed By The States, Provinces and Naupa (http://www.missingmoney.com/) . It's a good idea to do a search on every state you ever lived in to see if there are abandoned assets in your name.