whare1 wrote: I have no idea what the hell I have done this time
You killed your "Window Decorator"
Go to "Beryl Manager" and select a Window decorator, e.g. Emerald, Aquamarine, Compiz GTK Decorator ... depending on what you installed or not, some options may not be available to you.
If necessary re-run the "Emerald Manager" and pick a different Emerald theme; maybe you picked one your machine can't handle (e.g. too many transparency effects? I had that too once ...)