If you have any paperwork, then I would look at it and contact the plan administrator or hire a lawyer to investigate.

Apparently the Frontier was bought and destroyed by the Israeli property group, El Ad Holdings in New York. That might well mean your money has disappeared overseas forever. You could try contacting them, though.

If your 401K was invested into a legit United States 401 K plan, you will need to remember what company (Fidelity? Wells Fargo etc) managed the plan. If it was an internal program only, you are probably out of luck.

If a legit account was set up it still exists and has nothing to do with Frontier or the company that bought Frontier.... the 401K and the money belongs to you...

Contact a fellow former employee and ask them who your 401 K plan was through. Then begin contacting them... sometimes the 401 K companies are bought out or taken over by someone else...but you should be able to find that information out.

Eventually you'll need to prove who you are and that the account is yours (social security number for instance).

Good luck.