Trouble shoot Taichi x370 problem

Hi all, hoping to get some help. I've had my system (Specs: CPU: Ryzen 1700 Mobo: Taichi x370 RAM: GSkill FlareX 3200 GPU: Vega 56 PSU: Corsair CX 650W) for about 2.5 years now. Everything has been fine. Two nights ago, I wasn't able to turn on my PC. The lights flash, fans start, but then they immediately turn off.

Today I took everything off, stripped it down, reseated the CPU, and then built it back up one piece at a time. Each component I added, I jump started to check that it would run -- it did. After finishing it and getting back into the case, it still ran fine. At some point, the same issue happened again, however I removed the Power Button pins, and jumped it again and it worked fine. I was able to use my comp for a couple hours.

I came back to it tonight, pressing my keyboard to start it up, and it did the same thing -- just flashing lights and fans and then went out. I went to jump start it again, but now that's not working to get it to start.

(I added that keyboard tidbit, because I tested my PC after using it, by having it sleep, and then starting it from the power button -- that worked. When I used the keyboard though, it didn't work.)

Is this a MOBO problem, or something else? I'm confused by how I was able to get it up and running and use it for a couple hours, and now it's back to where it was before.

Any advice would be great!