Dell Inspiron 518 with no power

My Dell Inspiron 518 was working normal until last night when I successfully turned it off before going to sleep. I remember last months case when my screen was facing flickering issue; and so I had replaced the power supply unit to make sure it isn't the case. At first it didn't affected to the problem but after few days, my computer started working all of a sudden. So I don't think this is related to my problem. This was just to inform you a little history of my computer. I have tested all the components of my computer except motherboard and nothing seems unusual. So what can be the cause of the problem?

Try using green wire of 24-pin ATX connector to the motherboard. This will help you to know if this problem is related to motherboard or power supply. Just ground it to pin the power supply to start. Probably it should come out the PSU problem. In extreme cases it can also be due to cables fault. Moreover, do you have a UPS? If not then get it to avoid any such further problems.

Do you have any error messages or atleast beeps? What about the LED power button: Steady Blue, or White, Blinking Blue, or White, Steady Amber, or Off? If you are sure about your power supply then check your capacitors near video chip. Maybe something is getting short. Also check your fans, if are getting power because computer won't start mostly due to overheating.

I had some similar problem but on another computer model. But I think since the problem is similar, so my solution should work in your case. To check the motherboard, I connected a jumper wire to the ground of the monitor jack. The motherboard got the power. I then found the ground of the ac adapter. So motherboard is fine. I then tried the same thing only to the ground side of the motherboard at the ac adapter jack. It did not power up. So this states the ac connector jack is faulty.

Most probably the problem is related to the socket that is broken from the motherboard. It is common to happen depending upon your system handling. However, do not worry because the socket can be re-soldered or replaced easily if you have any electronic skills or you can even ask a hardware guy to do it for you.

It is likely to have dead or faulty motherboard. Try reinstalling the hard drive and memory without connecting the AC adapter. Press the power switch for some moment and then re-attach the AC adapter and see if the computer boot ups. If this doesn't work then try contacting Dell's Support team. This is a reliable, yet expensive, solution.