Computer problem

So, I stole my brothers 'dummy top'. Dont ask, he's a strange child. Anyway, he pounced on me to get it back and as he did so he pressed something on the computer by accident.
So now mibba is full screen.
No bottom bar. No url bar. I have to scroll over the top to get the url bar.
Does anyone know how he did it? and how to change it back to normal?
Please and thanks :)