pc power supply not working

hi
yesterday my pc was randomly shutdown, and never wake up, so i opened the cpu cabin for inspection. i found that the motherboards green LED is on. but else is not working, and power supply fan also not spinning. then i changed the power supply with new one, now the pc back to work. now i want to repair my bad psu. please can anyone help me whats went wrong, if the psu is dead then how and where from the motherboard green LED getting power.please help m.e i have knowledge of electronic basics and etc..

Thanks in Advance!

Meccawi

Seriously, if you have little to no electronic troubleshooting experience I would toss out the old PSU. It's not worth risking electrical shock or blowing up a component in your face. PSU are cheap enoough in cost that you are better and safer off buying a new one when the time comes.