Dreaming of opening a lock with a key is both positive and negative sign as per dream meaning and interpretation. The dream means you will find solution to a pestering problem. It also means success. Dreams of lock with a key also mean you finding secrets about someone and it will open up a huge problem in life. It also means you will find out about something that was hidden from you.
Dream of opening a lock with a key and you are doing it in the dream means you will open things that were closed for a longtime. It also means discovering new information or finding out about a family secret.
Dreams of opening a lock with a key and you are not seen in the dream means you will be investigated for something. It also means in your absence someone attempting to find out your secrets or things you have hidden.
Dreaming of opening a lock with a key and it is not getting opened means you will make several unsuccessful attempts to achieve something. It also means frustration.
Please not that watching treasure movies or anything to do with opening a lock with a key during daytime on screen or reading etc and then having the dream has no meaning. It is a mere extension of an intense daytime activity. The dream will have a meaning only when it happens without any daytime influence.