don't worry! these things happen! just make sure you do amazingly in your A2 exams (perhaps retake some AS modules too), take a gap year, and apply to university once you have your actual final A Level grades - rather than applying to uni with the predicted grades based on your AS Levels.
I know loads of people who've had to do this - and a lot of people manage to retake AS modules whilst doing A2 exams. AS will seem so much easier in comparison to A2. It will be ok!