boots to a black screen with a grey lock.

When booting up my mac mini goes to a black screen with a grey lock. I can not get past this because I can't remember the password I set last summer. I am unmade to any else (tied resetting PRAM and going into safe mode) but that does nothing and I keep getting the same black boot up screen with a grey lock. Before this started I was able to log into my machine, but would get constant popups of messages wanting me to enter a password that I could not remember, so I decided to try to put the machine into target mode to get what I wanted off of it, then I was going to reformat it. I was able to put into target mode but not able to get anything off it because it now only boots into this black screen with a lock on it.