I encountered the nvidia/kernel bug like others have posted, but mine occurred after I ran sudo dnf system-upgrade reboot.
I was able to recover my system by chrooting into it from a F38 usb and running a sudo dnf distro-sync --releasever=38.
Is there anything else I should run? The system boots into the new kernel and I have a GUI. I think I have a working, updated system.
Hello @awippler welcome to 
,
The reboot option simply tells dnf to reboot the system after the upgrade. You, obviously, were able to recover in the appropriate fashion. It seems that your system is working if you say you can boot into it. You could run uname -a in a terminal to verify the kernel you’re running.
That is not quite complete. After the download step, dnf will tell you Download complete! Use 'dnf system-upgrade reboot' to start the upgrade, and that is what the command actually does: set up some magic so that after the next reboot, the actual system upgrade is performed, and then it does that reboot (unless it is told not to). If everything works as expected, the system upgrade is triggered, followed by yet another reboot into the upgraded system.
distro-sync is the proper way to check whether everything is in order package/release-wise, and since you did that and the system works - happy hacking on F38!
system-upgrade is a dnf plugin to perform offline updates. Offline update means the update is applied when you’re not using the system, by restarting into a minimal environment first. This avoids certain risks with live updates (updates applied while you’re using the system).
The plugin always works in two steps:
dnf system-upgrade download ... downloads and prepares the update transaction, but doesn’t actually change your system. If you shutdown or reboot normally, nothing changes.dnf system-upgrade reboot reboots immediately and applies the update, then reboots again to finally use the updated system. Can only be used after dnf system-upgrade download ...