The Start and L keys are what I use to lock the laptop. The screen dims then shuts off. I use a TV as an additonal monitor and it says "No signal" until I move something around to get the screen back on.
If this computer was originaly windows Vista. You might have incompatible ram. Had a dell studio originaly with windows Vista. Tried windows 7 and windows 8.1 same problem. Changed the ram viola all gone. The ram that was in the computer was weird.