New build, was ok but now power issues

HI this is my first post so be kind lol

So i did a new build and all was fine for about 3 weeks, then one day my pc just restarted itself on its own, bottom line is it wont boot up at all and i was getting the "1 second fan spin"

I thought it was the board so i took that back for an exchange, same thing, bought another new psu, same thing, 1 second fan spin.

I found this site and went through step by step what was suggested and still the same, i breadboarded the mobo and just had the psu, cpu, monitor and memory installed still the same but then i saw how to power the mobo with a screwdriver across the power pins, this got the cpu and psu fans spinning and it looked like progress but there was no beep, i removed the memory, did it again and got 1 long and 3 short beeps.
I also got no power to the monitor either so cant see any info.

I have everything connected properly, the 4 pin 12v atx lead and the 20/24 lead all in place

Anyone got any ideas they would be most welcome, hope this makes some sense
My set up is

AsusP8H61-m mobo
Intel 620 processor
400 watt power suppy
2GB DDR£ memory

Thank you in advance