Boot up troubles with new pc

Ive just built a semi new pc. A process i have done many times before.

I installed a new motherboard, cpu, ram and psu.

When i try to start the sytem up the graphics card boots but then the power seems to be cut off.

I get no other boot information at all.
Ive tried dismatlinmg and rebuilding checking every connection twice and once again and no better
The manual is very un helpful

system:
Gigabyte GA 7S748 MBoard
AMD athlon xp 2400 266
512 ddr ram
and a brand new 400w psu
nividia riva tnt 2 graphics

PLEASE help its driving me up the wall