Did the upgrade go through correctly? Another user ran into this error here, and it looks like their upgrade was interrupted. You’ll need to reinstall the package to get dnf working again:
The “oh no!..” error is usually caused by some extensions that have not yet been updated to work with the latest Gnome version. When this happens, Gnome disables all your extensions, so hopefully you should be able to login when you try again—if it is an extension related error. You should be able to look at the journal logs to see what’s causing it:
They’ve already run the upgrade using Gnome-software here, so these steps will not help now that dnf is not working.