PC wont boot up.

Hi, first time post, hope its in the right place.
my pc recently switched off during the day when i was at work or so i thought but when i pressed the on button on the front nothing at all happened. i thought maybe the power supply had blown but i replaced that and still no sign of life, i have no idea what to do next.

the spec of the pc is:-
p4 3.2
1gb ram
512 nvidia fx card
200gb hard disk
any help would be great