Oh dear, it would seem, from your description, that this is not going to be easy to sort out for you. The lack of other parts, that you can swap and test with means, in reality, that we have to try and work out where the fault is most likely to be.
One final thing to check would be the pins on the APU. Is it possible that any of them got bent when you installed it? (I have done this before and had a similar problem) Carefully remove the heatsink and the APU and check to see if there is any damage to the socket on the motherboard or the pins on the processor.
If everything looks OK reseat the APU into the socket (it should drop in quite easily if everything's OK) and check that it is sitting properly before locking it into place and reseating the heatsink. Does this solve the problem?
