You should contact the Department of Labor. If you complete the online form:
https://www.askebsa.dol.gov/WebIntake/Home.aspx
Someone will usually call you within a business day to try to help you.
Additionally, you can try to find the Form 5500 (annual reporting form) for the plan by doing a search on https://www.efast.dol.gov/welcome.html
I find searching by EIN to be the most effective. You may be able to find your employer's EIN on your W-2 or paystub. If you do find one listed, I would save the most recent one and have the information handy or send it to the DOL with your request as it will have good information that they can start with, such as the legal name of the plan.