messing up X will require root access. so i guess it just messed up monitors.xml file which you can find from $HOME/.config/ folder

if thats what KDE is using also

Well , here's the output.
Problem is , I don't know what it looked like before :

[alin@archlinux .config]$ cat monitors.xml 
<monitors version="1">
  <configuration>
      <clone>no</clone>
      <output name="DVI-I-0">
      </output>
      <output name="DVI-I-1">
      </output>
      <output name="HDMI-0">
          <vendor>SAM</vendor>
          <product>0x0b43</product>
          <serial>0x5a5a4158</serial>
          <width>1920</width>
          <height>1080</height>
          <rate>75</rate>
          <x>0</x>
          <y>0</y>
          <rotation>normal</rotation>
          <reflect_x>no</reflect_x>
          <reflect_y>no</reflect_y>
          <primary>yes</primary>
      </output>
      <output name="DP-0">
      </output>
      <output name="DVI-D-0">
      </output>
      <output name="DP-1">
      </output>
  </configuration>
</monitors>

if you open up your GUI for configuring monitors in KDE, you should be able to change stuff from there and it will fill the file with info.. you can even delete(or make backup) of thisone and then try change from the GUI.

mine for example:
<monitors>
<configuration>
      <clone>no</clone>
      <output name="DVI-D-0">
      </output>
      <output name="HDMI-0">
      </output>
      <output name="DP-0">
      </output>
      <output name="DP-1">
      </output>
      <output name="DP-2">
      </output>
      <output name="DP-3">
          <vendor>DEL</vendor>
          <product>0xa0a4</product>
          <serial>0x334e354c</serial>
          <width>1920</width>
          <height>1080</height>
          <rate>60</rate>
          <x>2560</x>
          <y>0</y>
          <rotation>normal</rotation>
          <reflect_x>no</reflect_x>
          <reflect_y>no</reflect_y>
          <primary>no</primary>
      </output>
      <output name="DP-4">
          <vendor>AOC</vendor>
          <product>0x2710</product>
          <serial>0x00000735</serial>
          <width>2560</width>
          <height>1440</height>
          <rate>144</rate>
          <x>0</x>
          <y>0</y>
          <rotation>normal</rotation>
          <reflect_x>no</reflect_x>
          <reflect_y>no</reflect_y>
          <primary>yes</primary>
      </output>
      <output name="DP-5">
          <vendor>GRU</vendor>
          <product>0x4448</product>
          <serial>0x00000001</serial>
          <width>1920</width>
          <height>1080</height>
          <rate>60</rate>
          <x>4480</x>
          <y>0</y>
          <rotation>normal</rotation>
          <reflect_x>no</reflect_x>
          <reflect_y>no</reflect_y>
          <primary>no</primary>
      </output>
  </configuration>
</monitors>

edit:

i see you have nvidia card... if something messed up then delete that monitors.xml file or make a backup of it somewhere.. do a reboot and then set your monitors correctly from nvidia-settings and save them into X. if they are in X, no game should mess that up. Some games (SDL1 ones for example) will disable other monitors if going fullscreen but after you close them, system will restore the monitors that were disabled.

Yes , I know the monitor setting.
Problem is if i connect both monitors they both go offline.
And the DVI one is stuck at 800*600 with no option to adjust resolution.

I could try to put them in one at a time and edit the config file afterwards I guess.
Or maybe even SSH into the machine with both connected and see what the XML config looks like.

Is there an easier/dirtier option ? Like a reset of KDE settings ?

I havent used KDE much so i dont have answer for that. Did you reboot your PC after this issue?
cause sometimes all the different monitor config things might get into conflicts.. as there is nvidia-settings, monitors and even xrandr. SSH is an option also.

just in case throw your xorg.conf or 20-nvidia.conf from /etc/X11/xorg.conf.d/ into here also.

though i still doubt something got changed there unless you ran something as root.

alin@archlinux xorg.conf.d]$ cat 10-nvidia-drm-outputclass.conf 
Section "OutputClass"
    Identifier "intel"
    MatchDriver "i915"
    Driver "modesetting"
EndSection

Section "OutputClass"
    Identifier "nvidia"
    MatchDriver "nvidia-drm"
    Driver "nvidia"
    Option "AllowEmptyInitialConfiguration"
    Option "PrimaryGPU" "yes"
    ModulePath "/usr/lib/nvidia/xorg"
EndSection

OK , logged into Cinammon on a hunch.
Works fine. When I plug in the second monitor the image just pops up.
So this seems to be KDE specific.

System Settings --> Display and Monitor --> Check if a monitor is disable and setup the appropriate setup from there

Try to use ArandR so as to autostart on login and setup your monitors automatically just in case something broken your configuration. With ArandR on every boot you will be able to restore your default configuration,