x370 killer SLI/ac & Ryzen 5 3600 won't post

i would say bios then

I had the exact same issue. Same thing, Ryzen 5 3600 stuck in a boot loop, no video. Bios sticker on the mobo was crossed out with a sharpie. After some research (and reading the negative reviews on newegg) this board was marketed compatible with the Ryzen 5 3600 but it is not compatible out of the box. It requires an update to bios version 5.40. I read through all the compatible CPU's on the ASRock website for this mobo and focused on the ones that were marked "ALL" in the bios compatibility column. I then started shopping. The AMD A6-9500 was the cheapest so I picked it up for $66.45 USD (inclusing taxes and everything). I swapped out the ryzen for the a6 and it booted up just fine. I then had to update the bios in 3 stages. The bios sticker on the mobo said 3.20, even though it was crossed out it clearly read 3.20. Following the instructions on the site for this board you first have to update it to version 3.50, then to 5.10, and finailly to 5.40 which adds support for the Ryzen 5 3600 (among others). I swapped back to the Ryzen and she booted up first try! It was a pain, and now I'm left with this $66 A6 chip collecting dust but it works!