Hp pavilion dv6000 does not turn on

Question

hello my hp does not turn on when i press power button it seems like it begins to start all lights turn on i hear cooler but then it just turns off and then starts again i don't even see hp logo sometimes it starts up to the point where i can enter the bios but it cannot boot from hard drive because it cant find it. sometimes very rarely it boots to windows and goes without problems until i reboot or switch it off.