You should contact your previous employer's human resource department as soon as possible to inquire about your 401(k) plan benefits. Make sure you have your Social Security number handy to help locate records.
If you worked for a large company and either can't find a contact number or keep getting an automated response recording, try searching the web site GetHuman.com to find contact infomation that will get you through to an actual person. Also, if it's been a long time since you've seen any statements or received any communication, your former employer may have considered any balances you may have accumulated to be abandoned. Not to worry, there are resources for that as well. Begin by navigating your web browser to https://www.unclaimedretirementbenefits.com/ and perform a free search. Presuming you still have a 401(k) balance that's been left behind, you might want to think about rolling it over to an IRA.
I hope this helps.
Here is another place to look to trace your old 401k and pension plan.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 www.freeERISA.com. If you can find a Form 5500 on an old plan, it will have contact information.