A padlock on the screen means a firmware password lock has been enabled on the system. It also means that your internal macOS boot drive is not booting. When a firmware lock is enabled, it prevents the Mac from booting from any media other than the internal boot drive. So if you are seeing the firmware lock screen, it means the internal drive is not booting. See the following Apple article for details:
Set a firmware password on your Mac - Apple Support
FYI, the password for the firmware lock screen can be different than any macOS user account passwords. There is no connection between the firmware lock and any macOS user accounts.
If you don't know the firmware password, then you will need to take the laptop to an Apple Store or an Apple Authorized Service Provider along with acceptable valid proof of purchase and personal ID so that Apple can remove the firmware lock. I recommend calling ahead to find out exactly what paper work is acceptable for proof of purchase & personal ID. If you purchased this laptop used & you don't know the password, then there is nothing you can do since you cannot prove ownership.