PC WILL NOT START

Hello everyone. I left my pc on before going to bed yesterday. Today I woke up and it was as usual on sleep mode with the power button flashing. However even though I tried, I just couldn’t turn it on. I disconnected everything, let it sit for 30 mins, then checked everything inside. Aside from...

I switched from a i5-7600k to a i7-9700k. I put the 9700k in and cleared cmos, but the pc just bricked. I put the i5-7600k and clear the cmos and it starts up like nothings happened. Ideas? Edit: my motherboard is a asus z270h

I want windows as my OS, but I have never built my own PC before. I'm confused about what I should be buying for my OS. Do I have to buy it directly from the microsoft store? I want to buy it off Amazon for cheaper. Also, can I put it in a USB instead of using the disc?