How do you know "everything" turns on? Just because fans and hard drives spin up, and LEDs light, that does not mean the power supply is outputting all 3 required voltages properly. Without a display on the monitor, it would be difficult, at best, to determine all is turning on. I would check the power connections and if good, swap in a known good PSU.
Are there any beeps? Most cases don't have system speakers any more, and not all motherboard include them. So during initial assembly and when troubleshooting, I like to add a
System Speaker. Beep codes can sometimes be very informative. Plus, I find the single, short beep, indicating a
successful POST, reassuring.
Note that price is for 20 speakers!
Did you observe the necessary and essential ESD precautions when handling the CPUs, motherboards, and RAM? It is important to remember a static discharge from our fingertips can be so HUGE it torches a Grand Canyon size (microscopically speaking), scorched trench through an ESD sensitive device, yet be so tiny we (as humans) cannot see, hear or feel that a static discharge even occurred.
Again, I hope the necessary ESD precautions were taken to discharge any static in the body BEFORE even reaching for the CPU. As for reseating, it is important to remember that TIM (thermal interface materials) should never be re-used once the curing process starts (which is at first power up after application). The mating surfaces should always be thoroughly cleaned and a fresh new layer of TIM should be applied. And it should be noted a common mistake is to use too much TIM. The goal is to use as little as possible while still achieving thorough coverage. The most efficient transfer of heat occurs with direct metal to metal contact of the mating surfaces. The purpose of the TIM is to fill the microscopic pits and valleys in those mating surfaces, pushing out any insulating that might otherwise get trapped within. Any excess TIM is actually in the way and counterproductive to the transfer of heat process.
