Why have i gone stupid?

I'm serious. I have a really important exam tomorrow and i know nothing. I've been revising for the past year but i cant remember anything. I use to be really smart when i was younger, but somehow i've gone stupid? When i read books and poems the words are all jumbled up, i can't remember the meaning of lots of words and i've forgotten nearly everything in maths. Please, help me i'm freaking out!