First, contact Steam Support to see if they can help get your account back.
If that fails then I'd recommend contacting Frontier support about getting your CMDR back. However you can unlink your Frontier account from a Steam account by going to frontierstore.net and logging in, then heading to Third-Party Accounts and clicking unlink.
If you can't get your Steam account back, then depending on when you bought the game on Steam, you might be able to generate a new key to unlock the game on a new Steam account - as before 2019, Steam copies just gave you Frontier Store keys, so you don't actually need Steam to play. In that case go to the Partner Keys section of your Frontier store account. If your account was created after the system changed, though... I hope you have enough money spare