I know it may sound straight up idiotic, but check your cpu power cord. The other day I was messing around with some old Radeon HD5450 in my build, as well as my r7 240 and was about to throw it out the window from frustration. I was also booting windows 10, and couldn't figure out why it would start, but NOT turn off. I check all my cables, and here I had left the poor cpu power cord on the power supply, it was curled up like the binding of Isaac pose and weeping all over the PC! So like I said, just check the cord lol.