The power supply converts electrivity into low DC voltage to be used by yourcomputer. If your computer reboot without a warning or worse, it refuse to boot. Check out the power supply for any damage and inspect all connections.
First of all verify that the computer is connected to an electrical outlet that is working properly. If necessary, test it with another electrical appliance. In some cases a switch is sometimes present in the power supply; make sure its working properly.
A small red switch may also exist ( allowing you to choose the voltage).
Some power supply sometimes possess a surge protector,that prevent the computer from restarting after a short circuit. To solve this problem, simply disconnect the power supply to the computer, then press the power button down for several seconds to discharged the stored energy and to reset the protective device.
Provide that you have a spare power supply, replace the current power supply by a second, to determine if the problem aroused from this point.
Otherwise, you just buy a new power supply, taking advantage to buy a more powerful and more silent one
The block is usually set by 4 screws and power cords plugged into the motherboard, installation is quite intuitive, in order to prevent any connection error.