Question marked as
If your Mac is showing that icon then it means a firmware password has been set. This does not happen spontaneously. Someone did it purposely. Did you buy this Mac used? Botton line is, if you don't have the password then you cannot unlock the firmware. You need to get Apple involved and provide an original purchase receipt with the serial number on it. See the very bottom paragraph of the link provided by @BobTheFisherman.
