PC Shutting down by itself

[Melonator]

Before it was shutting down by itself, My friend was using his pc then suddenly power was cut for some reason and it came back, resulting in the pc being shut down. After that his pc started shutting down by itself. Any tests to do in order to know what is wrong?
I assume its probably the power supply, but it could be something else. Need a step by step procedure to find the malfunctioning part.

 

[redgarl]

It could be your motherboard to. Motherboard failures tend to power on and power down, again and again before posting. Power supply just act weird, don't power, or post and power down.

One thing for sure, the power outage had an impact and it could be one of the two.

 

[Melonator]

It could be your motherboard to. Motherboard failures tend to power on and power down, again and again before posting. Power supply just act weird, don't power, or post and power down.

One thing for sure, the power outage had an impact and it could be one of the two.

How will I check if its either the motherboard or powersupply?
Plus, his save file in terraria got corrupt, assuming that the hard drive was affected in the power outage