cpu not detected

Hi, hope you'll are having a nice day. My problem is that I bought a new Ryzen 7 5800X and a RTX 3080 to upgrade my system (Ryzen 7 3700 and RTX 2080 Super). When I changed the cpu and gpu and tried to turn on the pc it didn’t post, it only turn on all the fans (including the gpu’s, also the...

So I've been putting together my first new build with the following specs: Motherboard: MSI B450-A Pro Max CPU: Ryzen 7 3700x GPU: EVGA GeForce GTX 1080 RAM: Corsair Vengeance LPX 16GB Power Supply: Cooler Master - MWE Gold 750W ATX12V 2.31 80 Plus Gold Modular Everything is powering on, but...