Answers (2)
First: check to see if you are shutting down your PC and not restarting it, sometimes people make that mistake.
Second: your main drive could be corrupt, either by virus or because you did something incorrectly while working on your PC. This could be fixed by reinstalling your OS. Which would take a few hours and would erase any stored pictures, music or movies etc. But it's worth it if it gets fixed.
If it turns out that your main drive is corrupt, don't be quick to worry. You could replace your main drive with a new one and install your OS on it. many Hard Drives are cheap below 100 dollars, even the 1TB drives. You can find these Hard Drives on Amazon.com or a local computer hardware store, I would try Amazon first and do not buy used hard drives.
If you find that you want to install a new hard drive and need help, there are dozens of videos on YouTube (personally I like NCIXcom's channel best) that will show how to install a new hard drive the proper way, it's not as difficult as you might think.
If you have other questions about this, feel free to ask...
Your computer has a special chip (called BIOS) that holds software settings that enable the pc to automatically boot when power is supplied. This is useful for power outages so the pc comes back up automatically as is always available.
Generally the way to get to these settings is during a very short time window right during the boot up, you can hold down a key (many times the F11 but see your computer maker website for details).
Find the setting under 'boot-up options'. Change it to not restart if that is your desire. Save your settings and exit.