Are you implying that it comes on second time ? I know my woman's PC occassionally fails first time and we just put up with it. Must be something not powering up fast enough from cold I guess.
If it doesn't come on at all have you plugged in the power cable and switched on ?
Are there and lights on the PC, or any noise coming from it ?
Have you checked the fuses in the fusebox, plug, and PC to ensure the power is on ?