Computer will not start?

Hi, i have a computer that wont start. i plugged in the power supply again, and it has a motherboard light and the harddrive light. as soon as i push power, the fans spins for a millisecond and stop, and the power supply never turns on, i tried to push power again and nothing spins. so i just bought a new power supply, and plugged it in, and still doesnt work. please any help would be nice.