mac mini locked with 6 digit code, impossible to get code

Thank you it was helpful cause we didnt know what to do. We only found a half of the receipt, I'm guessing he had this mini for a while and then just let it sit . We are gonna try what you recommended an see what happens. We we're told that it was a system code that it has by others and that we hold down a bunch of buttons at the same time it would pull up another screen with a lock on it an we would type some code in that but we are not a computer genius we don't know what it was or what code to type anyway. But again thank you for your time and your reply