Computer nerds unite! For I have a computer problem.

[Loren Thomas]

I was once into building my own computers and fixing my own random software problems, but it seems my skills are dated. My problem is a boot problem on this computer. Everything worked great until my free upgrade version of windows 10 crashed, and couldn't fix itself. I had to create a boot file on a thumb drive just to get a fresh windows installation done. 

I can get it to boot, but only using F12. I don't even know what F12 does but I read it somewhere and it worked. It will not boot on its own, even though using F12 I select what is set as the 1st boot device, and it's the only one listed. This last time I disabled all other possible boot devices. Still won't boot without F12. 

Help?