If you have just replaced the power supply in your computer and it is not booting, you may want to clear the CMOS. By clearing the CMOS you will remove any settings that might have been particular to your previous power supply.

There are several ways to clear the CMOS: some motherboard have a button, others have a jumper, etc. The most common way to reset the CMOS is by removing the battery. The CMOS battery is the easily spotted coin cell battery (often called a watch battery) on your motherboard.

Motherboards vary, so we recommend consulting your motherboard's manual for a detailed outline of the procedure that is specific to your system.

There are also Jumper Settings on the Motherboard, please double check with your motherboard manual, Clearing the CMOS varies.

Here is a photo of a motherboard and the jumper setting to release the CMOS

If resetting the CMOS does not resolve your problem then you may want to try the Paper Clip Test.