Question Computer weirdness 101

[motherofboard]

I know 'monitor stuck in standby' is a widely questioned topic but please assess my individual situation:
I have 2 near identical basic systems that I built many years ago. System 1 is an ASRock Phenom2 N68-VS3 FX motherboard, running an AMD FX-6100 processor, through a Dell monitor. Ram is Dual 8GB DDR3 and 2TB HDD. No tricks, all onboard devices such as graphics etc. System 2 is an ASRock Phenom 2 N68C-S UCC motherboard, running an AMD Athlon 2 X2 250 processor, through an identical Dell monitor. Ram is 4GB DDR3 and HDD is 1TB. Again, no tricks, all onboard functionality. OS systems for both are Windows 7, both systems are near identical with the same programs, same setup, everything.
Both systems were working as normal, before I couldn't use them for 4 years due to certain world circumstances. This week I turned them back on. System 2 worked no problem. System 1 however, will not startup the monitor, which stays in standby mode. The tower whirrs away as normal, although the usb mouse lights do not come on, so assume is missing power. Nothing wakes up the monitor.

Action: I've literally been at this for 3 days. I have no hair left.
I start by attaching the monitor from system 1 to the tower of system 2, and it works no problem. I also attach the HDD from system 1 into the motherboard of system 2, and thankfully, it also reads as normal. Then I plug the monitor from system 2 into motherboard 1 and the monitor stays in standby (For the sake of testing I swap monitor cables/power cables back and forth, swap mouse and keys back and forth etc etc, still the same result.) My test method is always to return all connections, cables and devices back to the original setup in between trying a different swap/changing anything, as a control method.
Ok, I think, it must be the VGA port on system 1's motherboard, everything else seems to work fine when that isn't in the mix. I assess the situation, and decide that instead of resoldering a new VGA port on, I will instead take the motherboard (and processor) from system 2 and attach all of system 1's peripherals to that, since I know the VGA definitely works on system 2. (This means I can get the system working, resolder VGA port motherboard 1 in my own time, as well as the fact that motherboard 2, although lacking slightly in processing power comparison, has more functionality so I could actually hybrid all system peripherals such as HDD and Drives etc as one system-through motherboard 2 at some point in the distant future)
So, I do that. We now have motherboard and processor from system 2, with all other devices and connections from system 1 running through it. Aaand.. the monitor stays in standby! No joke, I go through all tests as before and the results are exactly the same as if nothing has changed. EXACTLY the same. Even the VGA port on motherboard 1(that is now connected to system 2's peripherals) now works perfectly. The new 'system2' works perfectly, as if no change has taken place, and still (the new) 'system1's monitor stays in standby mode.
Now. I simply do not get this, I don't believe what I'm telling you is possible, but there it is. I will forgo shouting out to you "how is this possible!?!" Because people I need answers and solutions.

Further actions:
Since that point I've tried everything I can think. I've removed both motherboard batteries and left them out over night. I've shorted the CMOS jumper cables whilst in operation, whilst in standby, whilst power is off. I've gone through system 1s HDD control panel for any display/sleep/system settings that may be out of order. I've hard reset the monitor both when in power and out of power (for the millionth time). I've bought a usb to VGA adaptor to see if I can bypass the VGA ports. Last night I went into the bios through HDD 1/screen2/motherboard 1 to see if there was some power issue/system fault that HDD1 was carrying with it (but of course this shouldn't matter for the fact it worked originally through sys2 and now through the new 'sys2'(-swapped motherboard), that, and the fact I don't believe the bios has anything directly to do with HDD functionality anyway.)
In any case, I have a BIG bug in my system. I simply don't understand what is going on here. To me it's literally against the laws of physics! Although am sure I don't need to tell the majority of you, it's exactly what computers are all about.. to those that know
Please don't hesitate to ask questions and I will answer as best as possible, but please read the above thoroughly to save me repeating myself as to what has happened here. I simply want to have tower 1 working with monitor 1, whatever the system entrails combination. Many, many thanks all

 

[motherofboard]

Amazing. 3 days of stress and then giving up and writing this post and voila.. I realised I hadn't swapped out the Ram during tests. Somehow, someway, the Ram was to blame. Took the 2x8GB ram out, swapped it with the 4GB and it works perfectly. Man alive. Computers.. should we ever dare 😭

 

[Misgar]

If the systems haven't been used for four years, chances are the CMOS batteries need replacing. If the CR2302 battery is flat (fully discharged), some motherboards will fail to POST. You won't see anything on the screen until you fit a healthy battery.

The terminal voltage of a brand new CR2302 battery will be roughly 3.3V. After a few years in a motherboard they drop below 3.0V, which is when I tend to replace them.

If you have a multimeter, check the voltage on both batteries. If the working system is above 2.5V and the dead system is, say, 0.2V, fit a new battery.

Another reason for zero output on screen, is when the BIOS is configured to use a plug-in graphics card, but you connect the monitor to the graphics port on the motherboard instead, or vice versa. If the BIOS graphics selection is set to Auto, it should detect whichever port the monitor is connected to.

I very much doubt the need to "touch up" the fifteen solder pads on a VGA port, unless you can see dry joints. I have several old motherboards dating back to around 2005 and they still work fine. If you do remove the motherboard from the case, look for signs of corrosion ("fur") on the solder joints underneath, if the computers were stored in a cold, damp location. Careful use of a stiff brush and isopropyl alcohol should clean off the corrosion.

 

[motherofboard]

Thanks for your response misgar. As I've said though have sorted it now. (See last post). Nice tips, although I did say that I'd removed the CMOS batteries (see post), and I'd been through the BIOS (see post), but you know, communication is a good thing! I hold my hands up, a didn't realise a stutter with the RAM would intercept the monitor startup, but in my defence I do believe my post was indeed pointing towards a VGAport refix, that was until I swapped the motherboards (see post). In any case, to those of a similar disposition, (and although I'd obvs removed the RAM and cleaned during the swap over), it was just one of 'those' Computer things.. now both systems are back up and running in their original configuration.. and apparently the RAM just decided to stop being a ****! Cheers again misgar. Really 👍