HI I recently built a system and it work fine then all of a sudden it would not start at all the power fans would start to spin for a second I had the power supply checked and I was told it was good so I assumed something happened to my cpu or motherboard anyhow that was not the case. I still beleive it is the power supply I check voltage to the 8 pin conector and the is none however if I check it as I hit the power botton it reads some voltage them drops off to none does the 8 pin esp12 volt conector suppose to have power all the time Thanks Wayne firstname.lastname@example.org
You can turn on the PC by momentarily shorting the two pins that the case power switch goes to.
You should hear a series of long, single beeps indicating memory problems.
Silence indicates a problem with (in most likely order) the PSU, motherboard, or CPU.
So, the best bet is to replace the PSU with a known good one of similar power capacity. Brand new, out of the box, untested does not count as a known good PSU.
Next best thing is to get (or borrow) a digital multimeter and check the PSU.
Yellow wires should be 12 volts. Red wires: +5 volts, orange wires: +3.3 volts, blue wire : -12 volts, violet wire: 5 volts always on. The gray wire is really important. It should go from 0 to +5 volts when you turn the PSU on with the case switch. CPU needs this signal to boot.
You can turn on the PSU by completely disconnecting the PSU and using a paperclip or jumper wire to short the green wire to one of the neighboring black wires.
This checks the PSU under no load conditions, so it is not completely reliable. But if it can not pass this, it is dead. Then repeat the checks with the PSU plugged into the computer to put a load on the PSU.