How do you know it's the Power Supply that isn't working?
It could be a number of things, as a computer needs a working Power Supply, CPU, motherboard, RAM, and Video Card to boot up (or Post). If any one of those things is bad, it won't work.
Do the fans start when you press the button? Any beeps? Or nothing?
A few weeks ago, my computer was working in the morning, and then in the evening, it wouldn't even turn on. Turned out my motherboard was gone and I replaced it, worked better than before after.
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