I am in dilemma

So i wanna, like if you just have look over you computer science(0478) exam on the exam day itself and realise that you are so fucked up with 15 marker that you know don’t know a shit about it. What do you do in that situation?? I am petrified if that happens to me. My teacher is pretty weird and just never resolves my problems so i just do all by myself. Please give tips and advice