Assuming all of the components are plugged in correctly, your switches are wired up, and the switch on the power supply is on, double check the wiring from the power and reset switches going to the pins on the motherboard. I've had a bad power switch before on the case. If the wiring is correct and the power button isn't working, try hooking the reset switch to the PW+ and - on the motherboard instead, just to test.
Other than that, start by unplugging everything except for the motherboard. (Just keep the main plug in and the CPU power). See if it does anything then.
If it still doesn't come on, get your power supply checked. You can test the power supply to just see if it turns on by removing the motherboard connection and jumping the green wire and the black one right next to it. The power supply should turn on that way. I'd advise against it though in case you make a mistake. If you try that, make sure no other components are is plugged into it. If the power supply comes on during testing than your motherboard is probably bad. Good luck. These things happen.
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