Logi Bear

Is the pc turning on but not showing anything on the monitor? Is so could be a old bios that doesn't support ryzen 3000 but could be a loose wire or ram or the GPU isn't slotted in correctly I suggest trying different ram or different ram slots then try turning on, takeGPU out and place it back in and try turning the pc on if nothing happens try taking the cpu out be careful tho cos the cpu can get stuck to the cooler so if your going to pull it pull it off straight up not at an angle that's what I did and the cpu was fine and then reseat the cpu and cooler and try turning on. 

Oki 

Ah oki doki 

Ah oki doki 

Depends which B450 board you have. Decent B450 boards will handle it without any issues. Cheaper B450 will struggle to run this CPU.

You’ll want something above a 6 phase vrm for reliability, so maybe something in the £120 range, but really it just depends I’d personally go and look at reviews.