cant start computer!!!

so i was writing some music on my music computer and it froze up on me. so i turned it off and left it for a few days because i was pissed. (i was on a creative roll!!!)
I try to turn it on this morning and nothing happens. so first things first i check to see if i bumped the cord loose or hit the switch on the power bar or something its all good i take off the cover and the little green light is on so i have power ive tried reseting the cmos and taking out the battery and well i dont know where to go from there.
the motherboard is a asus m2n4-sli its in a antec sonata II case with antec 450 watt power supply. windows xp sp 2
any suggestions would be much appreciated
thanks, J
p.s. is there a way i can start a computer besides the power button? maybe it is the button that has gone south on me.

 

You can try using the reset button, sometimes that will do the job, if it is the startup button.
Do you get any beeps, when it trys to startup.
Have you done the basic pc maintenance ?
http://forums.majorgeeks.com/showthread.php?t=106650

 

Have you done any cleaning of internals at all - blow out dust and fluff accumulated ?
Do any on-board fans start- cpu fan ?

 

thanks i will try cleaning it. just need to get some cleaner.

 

You should only use something the compressed air cans, but ensure the computer is cold when you do it, otherwise condensation from the cold air/hot pc will cause damage.
You can also unclip the ram stick(s) carefully, lift out and reseat again, as thie ram might not be clean around its seating- blow out slots with the compressed air.
If you are at all able to attempt any further maintenance, and have a wrist strap, or keep earthing yourself to a radiator, or house pipework,--It is worth checking the cpu, heatsink, and cleaning the cpu thermal compound off and renewing - I use artic silver.
Come back and ask , if you are unsure at attempting this, and usually, someone will advise further, if I am not on-line.

 

so i figured out it was the power supply but the power supply i used to test it i took out of my internet computer which wont start now! when i try to start it i hear a click in the speakers and the lighted fan lights for a split second. the computer i tested is a 24 pin connector the power supply is a 20 pin i dont know if that matters.
did i mess this power supply up testing the other computer?
help please! jason