PC WON'T BOOT: I have no idea what's wrong with it

[I know some stuff]

Hello,

I'm new to the LTT forum, so sorry in advanced if I posted this in the wrong category. I built a computer, and am having complications with it not starting up. First, here is the spec list:

 

Mobo: Gigabyte Z370XP SLI LGA 1151

CPU: Intel i7 8700,

GPU: GTX1060 6gb,

RAM: 16gb of g.skill ram clocked at 3000 mhz

PSU: EVGA 650 watt 80 plus GOLD

CPU COOLER: CoolerMaster Hyper 212 evo

SSD: Samsung 960 Evo 250gb NVMe SSD

HDD: 1TB Western Digital Hard Drive

CASE: Antec P100

 

     I am going to go VERY IN DEPTH, so you get the entire picture of how I built this computer (bare with me). 

    First, I assembled the CPU, GPU, CPU cooler, and RAM outside the case on the motherboard box to see if all the components worked. After assembling, I started it up, heard the beep from the speaker, and saw that it posted on my monitor. I, then, began to assemble everything in the case. After successfully assembling it in the case, I turned on the computer, and it posted again successfully. I installed Windows 10, set up all the drivers and installed all the necessary software I needed (Office, Steam, etc...). I continued using it for a week flawlessly; however, I noticed that it would boot loop sometimes on startup, and then, it continued to boot loop without being able to go into windows. I figured out that one of the two 8 GB sticks of RAM went bad, and  the computer would boot up with the one good 8gb stick of RAM. I got a new kit of 16gb RAM, and then the computer worked flawlessly for another week.

     UNTIL, one morning, I hit the power switch on the computer, and nothing happened. Not a single LED, fan, or audible noise came from it. I continued to hit it multiple times. Nothing. So, I figured that the power switch connector on the motherboard from the case came loose, the PSU died, some power connectors from the PSU to the motherboard came loose, or the motherboard is dead. I tested everything. No loose connectors from the PSU or case, and PSU still works (used paperclip method to test the unit). Luckily,  I have extra PSUs laying around my house, so I plugged another PSU in, and everything started up along with the mobo posting; 

     HOWEVER, IT SAYS THERE IS NO BOOT DEVICE. So i checked to see if the SSD was plugged in all the way....it was. I grab a hard drive with Windows laying around my house, hook it up to the computer, and the computer is able to boot and get into windows off of the hard drive. Using the hard drive lying around my house, I check to see if the NVMe SSD is showing up in File Explorer...and it is. I copied large files to and from the SSD withe ease, so it seems as if the SSD is working perfectly fine.

     So at this point, I have 2 problems: a PSU issue, and a booting issue. The EVGA PSU that I originally bought for the system does not work, so I have to use another PSU. The NVMe SSD by Samsung is recognized by the motherboard in the BIOS, but it will not boot (have to use other HDD to boot). I have no idea what is the source of the problem is because when I tested the PSU by itself, it started up, and when i tested to see if the SSD showed up, it was there and worked fine!

 

    Please help! And sorry for the long post....wanted to get every detail in, so there would be no confusion.