Sony vaio

Hello,
I got a problem witch actually cannot explain. But I'll try. When I switched on the laptop first the screen stays black and nothing works. But when I randomly keep pressing the buttons using my all fingers then the screen comes alive and then stuck on the screen wich we have to select start windows normally or diagnos the system. And then also I when I did the same thing using my fingers sometimes it comes to the login screen and stop or nothing happen. At this situation none of the keys are working . Sinetines the laptop works fine . I don't know what the problem is. Pls someone help me out.....,