Best place to start would be to reach out to your HR department. This is something your plan administrator will have a record of. The plan administrator could be a large company or a local company.
First step and most likely last step will be to reach out to HR and ask them this question. If they look at you like you have lobsters coming out of your ears, then ask who the plan administrator is and then call them.
Hi there thanks for reaching out. You need to contact the human resources department of your former employer or the investment provider of the 401(k) plan. Do you still receive statements or know how to contacts these entities?