Check the connections, but most likely the power supply is bad.
Unplug the computer and then plug it in and turn on the switch in the power supply (if there is one). Do you hear for a few seconds a fan? If so, power supply should be find, check the connections to the motherboard.
There are 2 possible solution for this problem:
1. Check the power supply. If the power supply is okey then
2. Most probably the motherboard is every broken or out of supplied power.