It'll help if you can be a bit more specific about what motherboard you're trying to install. There are specific problems. For example, if this is a newer motherboard, did you remember to plug in the special four or eight pin power supply connector for the CPU? This is distinct from the 20 or 24 pin motherboard power connector for such motherboards.
There are large differences depending on the class and type and brand of motherboard. Details, please.