So I was making a few upgrades to my PC, which included:
8gbs of ram (4 x 2 sticks)
2 320gb HDD.
I turned the switch to OFF and stuck the RAM in there, then put it back into the ON position, booted to BIOS and checked the RAM. It all checked out.
Putting it into off position again, it didn't seem to "click" and just sprung back to the ON position. I continued and installed the HDDs.
Now, all work done, I go to boot again. Nothing. The power indicator is not lit and the PSU Override switch repeatedly flips back to ON.
What could have possibly happened to my PSU, I carried out the install with all care required and now my PSU refuses to work!
I cannot find anything on this after repeatedly searching for an answer.
There are two switches involved in controlling power to your computer. You are using names some of us don't recognize, so I'm going to start with definitions.
On the front of the case is an On / Off pushbutton, which I THINK is what you mean when you say "the switch". It does NOT stick in when "OFF", nor pop out when "On". If you start from a computer that is off and push that button for a second or so then release, it will pop right back out again to its normal position. Plus, the computer will respond to that momentary contact created in the switch by starting up all power to your system so it can boot. That gets it all running.
When your machine is running already, the front pushbutton has two possible uses. The simple one is if you push it briefly (like a second or so). In response to that, the mobo will do one of two things, depending on how your BIOS is set. It either can ignore it, or it can put the mobo into a sleep mode in which stuff is stashed away safely on the hard drive and most systems have power shut off, but some key components keep their power so that the machine can be re-awakened quickly by another button push. The other action is if you push the button in and hold it in for at least four seconds. That will force the system to shut down immediately with no attempt to save anything or close applications cleanly, etc. It is useful as an emergency shut-down for a frozen system, but it does risk leaving a few corrupted files if things are not already closed properly.
For the most common action, that of starting up the machine with a short push of the front button, the mobo actually is NOT completely "dead" when the machine is turned "Off". I keeps a very few circuits alive and a software loop in the BIOS operating just to check for the push of the button. (There are also options in BIOS Setup you can set to restore power on a few other events, too.) So the mobo does actually consume a little power from the PSU on the normal "Off" state, and the PSU is NOT completely "Off".
The second switch is a rocker type on the back of the PSU itself. This is what many would consider a "normal" switch. It clicks from one position to the other, and is either On or Off. This one really does completely control the power into the machine. In its "Off" position it allows NO power to the PSU, nor from it to the mobo. This appears to be the one you called the "PSU Override Switch". When you turn it "On", the machine does NOT start up. It just gets enough power to the mobo to start watching for the front pushbutton to be pushed.
Now, you say, "the PSU Override switch repeatedly flips back to ON". Does this mean you actually pushed that switch mounted on the back of the PSU to the "Off" position and it refused to stay there, flipping back to "On" immediately? Or, did you just assume that it should be Off if you turned the machine off from the front? If the switch does not snap to either the Off or On position and STAY there, the switch itself is faulty and must be replaced. But f that is not what's happening, the switch may be OK.
I have had a few incidents that happened like this: I am doing something that goes wrong and I end up shut down, either by what looks like a normal shut-down or because I forced it down with a 4-second hold of the front pushbutton. But when I go to start up again with a front button push, nothing happens - it will not start no matter what! What I found is that, if I pull the power cord from the wall OR if I shut off the switch on the back of the PSU to remove all power, AND then I leave it unpowered for at least 30 seconds to a minute before restoring power, then it will work again! The push on the front button starts everything up normally! I hypothesize that in some odd circumstance the little button-monitoring function inside the mobo BIOS gets locked up and it fails to read the button action. But turning off all power and keeping it off for long enough (for the PSU's capacitors to discharge to no output) and then restoring power causes the entire machine to reset itself to normal operation. You could try this.