1. After ensuring that the problem is not related to the PSU or the display card, it may be that the RAM modules are mismatched or are not seated properly. If the problem is with the RAM, you will most likely hear beeps emitted by the BIOS. See the BIOS section to understand beep codes and how to troubleshoot the BIOS. Check the RAM modules to see if they are seated properly. Also, try to use modules from the same manufacturer. In some cases, RAM modules from different manufacturers don’t work together. Also check the memory section in this guide to troubleshoot RAM and memory.
2. The CPU may not be seated correctly. This problem should only be relevant if you have installed a new motherboard or upgraded the CPU. Visually inspect to check if the socket CPU is sitting flat in the socket, which means that the heat sink should be perfectly parallel to the motherboard. With a new socket CPU the heat sink may completely cover the CPU and you will have to remove the heat sink to check. Check for any crushed or bent legs. If the CPU’s socket locking arm was not raised up all the way up before seating the CPU or if it was not lowered all the way down after seating it, the CPU won’t sit properly. If the CPU doesn’t sit properly after this, then either the socket is faulty, or you have the wrong CPU for the motherboard!
It’s pretty difficult to tell by visual inspection if slot CPUs are seated properly. When in doubt, reseat the CPU, which is fairly easy since the heat sink and the CPU are an integrated unit. Also, make sure that you correctly identify release levers located on the top of the slot CPU package.
3. There’s no power to the heat sink fan If the heat sink has a fan, it should be hooked to the correct power source on the motherboard for the BIOS to monitor and control its state. Be careful! If you have installed a new CPU and switched it on without the fan it may have failed already! Hopefully, you will be lucky and get away with it by hooking up the fan. It may also be that the power point on the motherboard is failing.
4. The jumper settings may have been incorrectly set if you are experimenting with overclocking or have installed a new motherboard Return the motherboard to its default settings. Use the motherboard manual to get the default values.
5. The cabinet or uneven fixing of the motherboard may be causing a short. Put the motherboard on a cardboard box covered with foam or a static free bag and hook it up. If the PC boots normally then the problem is most likely related to the cabinet or the fixing of the motherboard. Mechanical stresses on a bent motherboard might be leaving a circuit open and preventing it from working. Fix the motherboard properly in the cabinet and if the problem re-occurs change the cabinet.
Note: If there is a crack on the motherboard, you are in serious trouble! The motherboard is a printed circuit board and hence a crack will break the circuitry. You will in all probability have to buy a new motherboard.