Contact the human resources department of your old employer.
Or, you can search through the Department of Labor, Employee Benefits Security Administration, for free.
Or, you can try a free ERISA search. You can search old 5500 filings for the name of your former employer at various free ERISA search websites. If you can find a Form 5500 for your plan, you should find the contact information.
Or, you can check with The National Registry to see if your former employer has listed you as a missing participant. The registry is a secure database listing retirement plan account balances that have been left unclaimed.