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What are your system specs?
What make and model power supply did you remove?
What make and model power supply did you purchase?
What are the detailed specs on the new power supply (not just wattage but all of the Amperage specs at the different voltages)?
Why did you change it?
Was there an apparent problem you were trying to solve?
Are you running the stock graphics or did you add/change the graphics card?
Do you get any warning or errors?
Are you sure you plugged everything in? Correctly?
Did you ground yourself from static before touching any internal components?
Did you need to (or accidentally) move, remove, or shift any other component?
Are you sure everything is seated properly?
Did you remove or shift any heat sinks even slightly?
Post back all answers and details for any kind of answer to your question.
You should have posted in the hardware forum... Don't be surprised if it is moved.
You have to be a little bit crazy to keep you from going insane.