Hi Luke 78
sorry to hear you are having this issue.
Just in case you have not got one below is a link to your MB manual.
Check that the CPU fan is plugged into the header on the MB and that
all other connections are secure then attempt to boot up with the
side off the case to check the fans are working correctly and in
particular the CPU fan, did you use any thermal insulating material
such as Arctic Silver when fitting the CPU, heatsink & fan (got to ask).
Some MBs have thermal sensors which will shut the system down to protect
it from overheating and this is possibly what is happening.
Other things to check are that the front connections to the MB are all
on the correct pins and that nothing is shorting the board out such as
a stray screw or bared wire.
If the above all checks out I would do a barebones set-up outside of the
case on a piece of cardboard larger than the MB, just attach the video,
one stick of Ram and the keyboard, the idea is to see if you can get the
BIOS screen, also if you used standoffs make sure they are in the right
locations/only where there is a screw hole.
Have you got or can you loan another psu to try?