What is the best possible reason behind this statement; when we start computer system and suddenly computer system switched off automatically?

MOTHER BOARD SHORTED WITH CASING OR POWER SUPPLY'S IC BURNED OR FAULTY

if ur system on and off means power is ok Butt if screen on and its black means HDD If Not A black Screen Means Board RAM Processor every system has a different Diagnoses these r the main

It seems Mother Board

A computer that turns off without warning is usually caused by a heat related issue.
processor, or other device gets too hot.
Check Processor fan moving or not

heat sint but sometime the os is giving the same isuee

Of the most important reasons that lead to the closure device directly after batch   - The hottest processor with a high degree, the lack of sufficient cooling.   - Defect in RAM installed.   - BIOS.

I'll start from checking the Power supply.

It's happen due to only Electric Problem, means power cable issue.

this is a ram problem , bco'z when we start computer then ram cant support to to system & then system will be automatically restarted ,

first check for power button, then power supply

There are many reasons regarding your question like

1- BIOS.

2- RAM.

3- Processor Heated.

and there are some computer hardware have cover sensor may be the cover not well closed.