If you can get another good unit plug it in from outside the case to see if it powers up the fans.If that is not an option and you can get hold of a volt meter just check one of the connections like to the CD/ROM drive. Or, this is Rather drastic but will tell you which part has failed.
How to bench test a computer that will not start when all other options have failed.
Sometimes it is necessary to bench check a computer that will fail to start. This involves Pulling the Power Supply and Pulling the motherboard. Remove everything from the motherboard and place the bare board and Power Supply on a piece of styrofoam or cardboard. With the power supply on the table connected to the motherboard and Power supply plugged in, Use a small tool to momentarily short the two power on pins together. The Power supply should turn on and stay on noted by the supply's fan running.
Power off. Now put the CPU/HSF on [easier to do when the motherboard is out of the case], add one stick of RAM, and mount the motherboard back into the case.
Connect the Power Switch wireing, no mouse, keyboard or monitor and again see if the power supply turns on and stays on. Get past this point power off and connect the keyboard, mouse and monitor, If needed Get into the BIOS to set boot options. Add one thing at a time checking each item. First drive to add should be your CD. drive then hard drive.
I have a home made and it currently will not power up. I will need to test all of my components but feel the power supply is the first place to look. Has anybody had this problem and how did you go about solving it?