Secret Keys control the funds sent to individual Stacks addresses. Secret Keys are either a 24 word secret phrase that users were prompted to record in the process of generating the Stacks wallet, or secret keys connected to a hardware wallet device. A full overview and background on Secret Keys is available here.
In the past, users were able to use either a Trezor or Ledger to set up a Stacks wallet address as an alternative to generating a new secret key for their STX holdings. As such, they may have 12, 18 or 24 word secret keys that are associated with their hardware wallets that might have been used to set up the Stacks wallet originally.
Users that think they may have lost their secret key are encouraged to first check any hardware devices for their holdings by connecting to the latest Stacks wallet for desktop via hiro.so/wallet and trying the steps for Ledger here or Trezor here.
Users are also encouraged to check any safes / password managers or paper files with important information to find their 24 word secret keys if they have never used a hardware wallet.