Before you go out and buy a new power supply, pull out the power cord and pop in a new fresh one, or one off a different computer. Sometimes, and i have no idea why, the cable(im guessing) gets shorted out and simply removing it or replacing it will allow you to power up. If this doesnt work, i'd say get a PSU tester and see if the power supply is really blown. You can also check on the motherboard to see if any of the capacitors have "popped".