I replaced installed additional RAM in my eMachine

I replaced installed additional RAM in my eMachine T4510 but it wouldn't complete POST. Removed the new RAM and re-installed the factory-original RAM and it still wouldn't complete POST. Removed all RAM and the keyboard lights flash, as does the FDD light, but doesnt complete POST or even beep that the RAM is missing; monitor never lights up during POST.I left EVERYTHING unplugged overnight, plugged in only the power cord, keyboard, and monitor (RAM still removed) with no change in symptoms, What do I do??