Displays Monitor, mouse and keyboard aren't working

hey guys this is my first post so go easy I'm no pc guru, so after openening my machine and removing ramm and replacing it (to check which ddr it was), I powered back on my pc to find no power to the monitor,mouse or keyboard the fans still power up and I hear no beeps. Im at a loss as what can be done and I need to get my corusework off it. My machine is a packard bell ipower x2.0 model ipwG5620 with a nivida gtx550ti card in other then its the shop standard one, any help would be awsome.
regards