|You replaced the power supply 3 months ago and just replaced it again or you replaced it once 3 months ago and are still using that one?|
If it is the power button you can open the case and follow the 2 wires that go from the button to the motherboard. Then remove that 2-wire connection from the motherboard pins. Then with the power cord attached as with normal operation, take something like a metal screwdriver and carefully touch those 2 pins together for a second. If the PC starts up then likely the power button is the problem. If not, it's something else.
If it doesn't start up, try temporarily removing all the cards except video and disconnect all the drives. Then try starting it again. If it starts up then one of the removed or disconnected items is at fault.
If it still doesn't work you're pretty much left with the power supply or motherboard. Often a power supply will fail and ruin the motherboard as well. If you're using a brand new power supply then the motherboard is likely bad. If you're still using the one you replaced 3 months ago then it could be the power supply, motherboard or both that are bad.
You're not really green until you're soylent green.