Question Computer refuses to boot after "swap" of hardware

[kocothehooman]

So i have these two old computers
One is an office pc with a better processor (but unfortunately on a different socket)
The other was my pc for a long time
So a few months back i changed it out, and decided to give it to my sister.
Before giving it to my sister i tried multiple times to change the motherboards, but it never booted, so i just put them back together. Until today i thought it was a BIOS problem, since it refused to go into BIOS (there was a boot screen and an operating system boot screen with a constantly flashing underscore).
I took the hard drive from the office pc and put it into my sisters.
Until today i had swapped back-and-forth the MBs with no problem. Today however was different.
Today i thought that it could probably boot now and i swapped the MBs again. It didn't work, so i put them back together.

1. I was able to get into PC info and saw a BIOS revision date, which confused me
2. Upon pressing the key to enter the BIOS it beeped once(not a high-pitched one, a deeper, quieter beep), and then just stared beeping continuously(with like a <1s delay between each beep)

After swapping them back together, it refused to boot.(unfortunately i neither have a speaker, nor a led screen on the motherboard).
Both GPU and CPU fans are spinning.
I have only video,keyboard, mouse and power plugged in, but there is no video, the sensor of the mouse follows this pattern (. is flash, - long flash, a space is nothing)
.....-- ....-.....--
Keyboard lights for caps and numlock dont update.

 

[USAFRet]

Swapping hardware, in particular the motherboard, often/usually required a fresh OS install.
Not booting up is not uncommon.

 

[kocothehooman]

Swapping hardware, in particular the motherboard, often/usually required a fresh OS install.
Not booting up is not uncommon.

forgot to explain
with the office motherboard it refuses to boot into anything (i tried video and power only with an installation disk it refuses to boot, same with mouse and keyboard)
now it refuses to even give boot screen (screen is fully black, and the light on the button indicates no signal)