Worked fine last night, no updates have run or anything. Where to start diagnosing this? In a way where I won’t need a keyboard?
Okay, couple routes to get in. It sounds like whatever you have going on is probably some setting specific to your graphical environment and account (a KDE keyboard setting?)
root
If Fedora lets you log in as root and the root account isn't affected, that could get you to a point where you can affect the system.
onscreen keyboard
If you want to try working from inside your account, I suspect that KDE has some sort of onscreen keyboard, which would temporarily let you use your mouse to fill in for the keyboard, though I don't use KDE. For GNOME, it sounds like it's at "GNOME Settings > Accessibility > Typing > Screen Keyboard".
just use the mouse and copy-paste
I have no idea how KDE Plasma sets things up, but traditionally on X11, and for me still on Wayland, you can select text to put it into the PRIMARY clipboard, and then middle-click to paste it somewhere. So if you can get selectable text with letters up and that's available, that should let you input text.
use the mouse alone
I'm guessing that most likely this is some KDE keyboard setting specific to your account. So if you can get into your keyboard settings with your mouse
can't help you there, don't know how to navigate KDE
you could pop into there.
console rescue mode
You very probably have a "rescue mode" or some such option in GRUB, the bootloader that comes up initially after any BIOS screen when booting. I don't know whether current Fedora shows GRUB's menu and gives you a couple seconds to hit a key as GRUB shows up, but I believe that holding shift during boot should show GRUB if Fedora hides it. On Linux distros, that'll tend to get you logged in as root on a console, though I haven't used Fedora in a long time.
Whatever config is affecting you may not apply in that mode.
init=/bin/sh console mode
If Fedora doesn't have a rescue mode, you can boot into an extremely primitive mode where barely anything is running (e.g. Control-C won't work to kill programs, you have one console) by stopping the boot process in GRUB and then editing the kernel command line and appending init=/bin/bash to the kernel command line. You'll have basically nothing but the kernel and bash running, but this will get a Linux system that has basically everything broken at least to a command line. Doubt that it's necessary in this case.