This forum is just for forum-related issues, so you'll want to repost in the PC Hardware forum where members can be of greater assistance.
When you do, please include the computer's specs, such as the make/model of the motherboard, CPU, and PSU. It could just be that the second power supply is underpowered to support your computer, so the details matter. Remember, the more information you can provide (Any error beeps? What is displayed on the monitor? Might there have been a power surge? etc.) you can provide the better informed our guidance will be.
My computer could not turn on, so I switched a power supply. But now, it only turn on for a few seconds and then turn off by itself. What should I do now? change the whole system?