Did you remove all cables from it along with the battery before you held the power button. By the way that only works for laptops. If it is a desktop and it doesn't power on, your PS is bad and you need a new one!
Well if you are telling me that you have removed the battery, removed the cables to the wall, held the over for 30 seconds, and nothing. Then it is beyond troubleshooting via a free forum. Take it to a qualified technician.