CPU and motherboard compatibility
You need to make sure the CPU is compatible with your motherboard. You need your motherboard make and model and possibly the BIOS version. With that, you can check CPU compatibility (usually on the manufacturer's web site). If you need to upgrade the BIOS, do so before you remove the existing CPU.
Compatibility with the socket and the chipset is not enough. If the motherboard BIOS doesn't know how to setup the CPU, it won't work. If the motherboard's VRM can't supply the CPU the voltage or power it needs, it won't work. And so on.