Improperly seated RAM will cause the system to not turn on. This is because RAM is integral to the computer's operation. Turn off the system and unplug the power cord, then re-seat your RAM modules.
Reseat the CPU. Make sure no pins are bent and that it's sitting properly in its slot. CPUs lock into the motherboard. Reapply any thermal paste before remounting the heat sink.
If all else fails, the problem could be the motherboard. Getting a faulty motherboard is not unheard of. This is typically the last resort; this should be done only if all the above steps are exhausted.http://hardwaretexpert.blogspot.com/2010/10/different-types-of-rams.html