I’m a bit dumb so I’m not sure what actually happened
when I start my laptop, I get to a blank screen with a mouse cursor and I can get into tty with Ctrl+Alt+F3
startx returns: (I typed this out sorry for any typos)
(II) modeset(0): Initialitising kms color map for depth 24, 8 bpc. Could’t exec startkde: No such file or directory. xinit: connection to X server lost
Waiting for X server to shut down (II) Server terminated succesfully (0). Closing log file.
It is possible that your system is too low on entropy when it boots. Try this… 
sudo pacman -Syyu haveged
sudo systemctl enable haveged --now
I still have the same issue
The system tries to load kde but kde is not found.
did I accidentally uninstall KDE?
I forgot to mention that you are not supposed to use the startx command. With the haveged installed, your display manager should come up completely and allow you to log in.
The reason why it didn’t work with startx is because you already had an X session starting up, and in order to start a second X session, you have to attach it to a different display port, like so…
startx -- :1
I don’t know - did you?
how do I find out
When I get the blank screen with a mouse cursor, is that an X session?
Yes, it is, but only in its initial phase, because then it’s supposed to load the display manager, i.e. the graphical login screen. And for KDE Plasma, that’s sddm. For XFCE, it’s lightdm. And for GNOME, it’s gdm.
Ok, but it never actually progresses it just stays on that screen. I left it for 10 minutes
Well, as @linux-aarhus said, it looks like your Plasma installation is damaged. You’re going to have to reinstall that, but as I don’t know how much of your Plasma is missing, nor how much of Plasma gets installed via the meta-package, you may have to do a lot of additional installing.
I suppose you could start with plasma-desktop. That should already pull in most of the needed stuff, I think.
I reinstalled both plasma and plasma-desktop
Also I accidentally marked your post as a solution but it still doesn’t work
startkde is old config, command is not in plasma package - now is startplasma-x11
old config : change command in ~/.xinitrc for to have a good startx command
it says:
$DISPLAY is not set or cannot connect to the X server
I edited ~/.xinitrc
Now when I start my laptop, I get a blank screen with a default KDE mouse
I enter tty3 and run startx and get my theme loading screen, then a blank screen with my theme mouse cursor
I still can’t get into KDE
Not a solution, but like I said, you’re not supposed to use startx. Manjaro is configured out-of-the-box to automatically start the display manager ─ in your case, sddm ─ at boot time.
So the fact that it doesn’t do that on your system demands further investigation. 
Oh yeah, I forgot. what ‘further investigation’ do I need to do?
Well, for starters, why your system is not behaving as it should. You must have done something to mess it up ─ no offense intended.
As I said I’m pretty dumb so probably yeah but I don’t know how to fix it