Hi there,
I assume you do normally turn your computer off and this is just an issue that has come up now?
firstly, does your computer turn on and then turn off after a couple of seconds? or is it more like a flicker of life which only lasts for a split second?
if it's the former, I'd suggest checking out the power button on your computer...
if it's the latter I'd suggest something has gone wrong somewhere on your computer, any changes made recently? if so, change them back, see if it works.
As a drastic method of working out what it is, you could attempt to remove everything from the pc, if you disconnect everything (hdds, dvd, cd) then you can rule them out, however i believe that if you take out the graphics card and the ram then the motherboard would still spin the fans (although nothing else would happen)
if that still causes issues then you can always turn your psu on without attaching it to the mobo, google it, it can be done, although i'm unsure how you would go about checking the psu under stress, you could certainly check that all the lines were the correct voltage.
anyway, have a look at what i suggest, maybe it'll help, maybe it won't! either way, it's my first post, so hello everyone!!
Mark