You have an HP a400n desk top that was working fine and then all the sudden it wont come on all that happens is the power button light flashes on and off Any idea what your problrm might be thanks?

Not an easy one to answer. Depending on how confortable you are with taking a PC apart you may want to bring it to a computer shop for a diagnostic. If you want to try to do it yourself heres how:1] always unplug before touching any internals2] visually inspect the PC for damage such as; loose wires and cables. Smell it for electrical burnt components especially the power supply. Any chance PC was opened recently by anyone or if there are kids in the house make sure the motherboard is not grounding out on a stray screw or extra standoff or yes even a penny.3]Take notes on anything you remove so that you can put it back properly.4] Strip the PC down to its basic components. Motherboard, CPU, 1 stick memory and video card and power supply. Remove all other items such as network cards, modems, ribbon and power cables to the drives.5] did it boot up? if yes slowly reconnect one device and try it again until it fails and you have found the bad device. If no go to next step6] test the power supply with a tester(they are not always accurate) or use a known good high watt power supply.7] if it still did not boot up you have narrowed it down to the basics and it is a matter of using a good PC to test those parts.