Computer restarting or closing down during activities.

[aluculeseir]

Hello, community of tom'sHardware. You guys have helped me a ton in the past with so many questions answered by the most of ye', and managed to help me figure out what was wrong with my system without me even needing to ask anything.
But now i'm stumped by some problems happening inside my friend's computer. It crashes, restarts and shutdowns during basic activities such as browsing, watching videos or working on a document or during more intensive ones such as gaming. Sometimes it goes out even when on idle.

And lately, it's been throwing errors after closing down such as:
"Missing operating system"
or failing to find a harddrive.

Both of those problems were 'fixed' by the old power-cycle trick I've learnt here, but they still came back in full force.
I believe it's the HDD's problem but I've came to seek your aid.

 

[hang-the-9]

Yep, sounds like a drive issue. Aside from just turning it off and one what did you try already? Did you scan the drive for issues? Re-seat it? This a laptop, desktop? Brand and model of components? Need some details.