Try reseating the control panel cable. Then try removing any unnecessary components, including ALL expansion cards, drives, and memory (except one DIMM in the first slot).
My powerdge 750 server is not booting, when AC current is applied the server the Power light flashes green (Meaning that the server has power but has not booted) when I press the power button nothing happens. I am in an urgent stituation, I need it going within 4 or 5 days. If anything needs replacement I need to know now. 
Poweredge Server 750 - Not Booting