Do you have access to another CPU board to swap over and try in the machine and see what happens with a CPU in there that you know is good?  This would be my first option so you know whether the board is dead or alive and go from there.

Re-seat all the connectors maybe and see what happens as well. 

Check switch contacts for corrosion such as the start button to make sure it is actually working, if it has been sitting for a while the contacts may have corroded or similar.