Its hard to tell you without knowing what cpu your putting in what motherboard. please post your full system specs.
Am2+ CPU in AM2 socket should work. my guess would be that since the board is older than the processor, it might not support the cpu with the current BIOS. you will need to goto motherboard X's website and search for a CPU compatibility list for that model, it should tell you which bios version you need to run that CPU.
However you will need a CPU which works inorder to do the bios update, so I hope your just upgrade the CPU and still have the old one around.
A no post is a very general problem though so it could be something different all together. For that reason please post all your components.