I lost my keys- I use this when, right now, I need my keys and I don't know where they are. It is the simple past. Yesterday, I lost my keys. This morning, I lost my keys. I lost my keys immediately after receiving them.
I lost my keys somewhere in my apartment, I know they are here.
I have lost my keys has more possible meanings. Present perfect is not now, it is before now, leading up until this moment. The action, to lose, occurred before this moment (otherwise, you would not know it occurred) and could occur anytime until the present.
I have a recurring problem of losing my keys. I have lost my keys so many times it makes my head spin. (it drives me crazy.)
I want to go somewhere, but I have lost my keys this week. (I don't know when.) You could use lost here, but advanced speakers use helping verbs (have, has) with the participle (lost) for non-defined moments in time.