This is one of those questions that can only be answered by,
Make sure all connections are made properly, all cables in the sockets are pushed fully home, all cards are firmly in the slots, memory sticks, graphics card etc. The cmos battery is correctly located.
The cpu cooler is fixed properly.
The motherboard is not shorting inside the case.
Try resetting the cmos (bios) using the jumper on the motherboard.
Reset jumper (see manual for details) and remove cmos battery and power lead.
Cycle on and off the power button a few times.
Reconnect the battery and power lead.
Replace the jumper to the original pin position
Reboot the PC.
andy