Custom built PC refusing to power on for no apparent reason

[up809212]

My (rather old) custom built PC won't power on. The computer was working fine, then went to 'sleep' mode, as set too (after no use 10mins). The next morning it wouldn't turn on. I unplugged every component from the mobo and reset the mobo with the jumper (touching the 2 pins) The PC then booted. I plugged every component back in one by one, booting the PC each time I plugged in a 'new' component. The PC then booted into windows for less than 30 seconds before shutting down. After the PC shuts down it won't boot back up until I unplug every component and reset the mobo. I've replaced the thermal paste on the heat sync, the i7 idles (in the bios) at 30-40 Celsius. I've also re-seated the i7. Now i've hit a brick wall (unrelated to re-seating the i7). I've unplugged every component and reset the mobo with the jumper. No power seems to be reaching the mobo or any components (a self defence mechanism?). I've ordered the mobo speaker, hoping it will give a beep code but that isn't arriving for a month...Any ideas what the issue could be?

My PC spec:
3rd Generation Intel Core™ i7 3770K 3.50GHz Socket LGA1155,
Crucial Ballistix Sport 8GB (2x4GB) DDR3 PC3-12800 C9 1600MHz Dual Channel Kit,
GIGABYTE GeForce GTX 650 Ti OC 2048MB GDDR5,
GIGABYTE GA-Z77P-D3 Intel Z77 (Socket 1155) Motherboard [ GIG-Z77PD3 ]
Corsair Gaming Series GS700 - 700 Watt 80 PLUS Bronze Power Supply [ CSR-GS700W ],
SeagateBarracuda 7200.12 2TB 64MB Cache Hard Drive SATA 6 Gb/s 7200rpm,
Samsung S222BB 22x DVD+/-RW SATA,

The PC runs windows 10 and loads every application (or used to...) at the click of the mouse. No component seems to be malfunctioning so I don't know how to proceed. Any ideas will be greatly appreciated!

 

[cin19]

I unplugged every component from the mobo and reset the mobo with the jumper (touching the 2 pins) The PC then booted. I plugged every component back in one by one, booting the PC each time I plugged in a 'new' component.

This is the right way to test.

The PC then booted into windows for less than 30 seconds before shutting down.

So which part did you plug in that causes the PC shuts down? that part has problem, you need to replace it.

 

[Peter Martin]

i am thinking the cpu is bad. if the thermal paste is applied properly and the cooler is working, then it should be fine. have to test and rule it out though, maybe a mobo problem even. also, can you try to boot a memtest usb and test the ram? if it shuts down and the cooler is working it's probably the cpu

 

[cin19]

I unplugged every component from the mobo and reset the mobo with the jumper (touching the 2 pins) The PC then booted

That means either cpu or MB works.