Hi,

What is the model number of the desktop?

Try disconnecting everything from the motherboard except the PSU, the RAM and the video card (if not using onboard video), i.e. HDD, ODD, any other cards etc. and see if it starts and if you can get into BIOS.

If the PC still doesn't start, try the paper clip test to see if the PSU starts OK. Here is a link that shows how to do it. If the PSU doesn't start using the test method it is faulty and needs to be replaced.

If the PSU does start OK, then use a DMM's (Digital Multimeter) Voltmeter function to measure the output voltages from the PSU. Also check with the DMM's Ohmmeter function the the desktop's Power button has continuity when operated.

If all the tests prove OK then there is a problem with the motherboard.