How exactly could the update of your installed operating system have affected the live image on a USB stick that wasn’t even plugged in when you updated your system?
Your profile states that you are using Plasma — which is probably part of your problem right now, and that is why I’ve added the plasma tag — and when it comes to Plasma, there are certain steps that need to be taken into account before applying updates, and especially so when upgrading from one major version number (Plasma 5) to another (Plasma 6).
Some of these steps have been outlined on the pertinent Stable Updates thread — among which, the recommendation to log out of Plasma and to run the update/upgrade process from a tty — but it’s a little more involved than that, and several recommendations have been made here at the forum — both on the aforementioned announcement thread and elsewhere — to empty ~/.cache/ from a tty while completely logged out of Plasma, and preferably also before updating/upgrading your system.
Another potential pitfall would be if you happen to have any third-party widgets, applets or themes that are not (or no longer) compatible with Plasma 6. The responsible thing to do would have been to remove all of those before doing the update/upgrade, exactly because Plasma 6 is such a major overhaul under the hood — it’s entirely based upon qt6, whereas those widgets, applets and themes were developed for qt5.
Without further information, and pending the results of your efforts at cleaning out your ~/.cache/ — and possible deleting or renaming all of your ~/.config/plasma* files and your ~/.config/Trolltech.conf file — I’ll leave you with these two links for now… 