Firstly whats the paper out of? If you missed 20 marks out of 100, not too bad. Out of 50? Well depends on UMS but probably no A for you.
You're probably just overreacting when you say you guessed half the paper. Chances are, things aren't as bad as you think they are. But still, no way to know until results day, I suppose. You could go crazy trying to pick everyone else's mind to find out what you got wrong, but I wouldn't recommend it.