PC Will Not Power Up?

Since installing this combo the computer has been failing to start
intermittently, until now it won't start at all. Previously,
unplugging the mains plug and reinserting it cured the 'no power'
problem, but now all I get is a quick burst from the PS fan and then
nothing.

When this happens the RAM LED lights.

Grateful for any advice on this one.

Board is Gigabyte GA-8PENXP and PS is Q-TEC 550w Dual Fan Gold.

Thanks.

P.S. My total failure happened after the computer had not been used
for some time. Like three or four weeks.