why does the Computer turn off midway while trying to boot up ?

Most likely a corrupted boot volume. It can happen as a result of an improper shutdown, such as a forcing a power down after freezing. Most of the time it will self-repair. When it doesn't there can be damaged components needed to boot. I've had it do what yours is doing which is just shut down, but others find that it might restart or stall indefinitely.

What machine do you have?