[SOLVED] PC Stuck on Boot screen - New CPU + AIO

Nov 1, 2021
Hi there,

I'm very new to this, but I've been slowly upgrading parts to my PC.

I have just replaced a Ryzen 3300 CPU with a Ryzen 5900X CPU, installed in an ASUS X570-E Gaming Motherboard.
I also replaced the stock cooler with an ASUS Ryujin II 240 AIO.

Afterwards when I attempted to boot, my PC was stuck at the loading screen with the ASUS logo, so I entered the BIOS and I could see a "CPU Fan Error!" warning as well as the CPU indicating there was "0 mhz" clock speed.
I set the CPU Fan to "Ignore" to see if the PC would boot, and it wouldn't. I reentered BIOS and the CPU was still indicating "0 mhz" although the "CPU Fan Error!" was gone.
I decided to remove the new CPU (5900x) and reinstall the 3300 to test if I had broken another piece of hardware or if it was the CPU causing the PC to not boot.

The PC has booted with my old CPU, and the AIO installed. Though I had the "CPU Fan Error!" again, and had to set the CPU Fan to "Ignore" again, to boot.

Does anyone know why my PC won't boot with my new CPU?
My guess was that my CPU isn't compatible with my Motherboard out of the box, but I'm a novice and don't want to break anything by trying the wrong solution.

PC Specs incase:
GPU: AMD Radeon RX 580
CPU: AMD Ryzen 5900x / AMD Ryzen 3300
AIO: ASUS Ryujin II 240
Mobo: ASUS ROG Strix X570-E Gaming WIFI
PSU: ASUS ROG Thor Platinum 1200W
RAM: G.Skill TridentZ Neo 32GB 3700Mhz (Running at 2133Mhz)