There are many reasons why a computer won't start, like hardware failure or system errors. Below are some of the reasons why a computer won't start and how to fix the problem:
Power Failure
The first thing you need to do is to check if your power supply's fan is working properly.
If you have just recently replaced your power supply, make sure that it is compatible with your computer. It is recommended that you only buy a power supply that runs the same voltage as your old one, unless suggested by an experienced technician.
Reseat Hardwares
Reseat power cables, memory card(s), expansion card and external devices as well.
Reseatting in computer is done by unplugging or removing a hardware component then plugging it back again.
NTLDR is Missing
The easiest way to fix the "NTLDR is missing" error is to use your Windows installation CD, you may be able to boot from this and use the Recovery Console to fix NTLDR manually.
Advanced Boot Options Loop
CLICK HERE if your computer starts but returns you back to Advanced Boot Options.
Blue Screan of Death
