Pick yourself up, dust yourself down and get back to work. Use it as motivation to do better at A level. Show your true abilities at that stage. Do not accept that any university is out of reach, even if it might seem that way now.
If you don't get into Oxbridge, that too is by no means the end of the world. It might seem like it now but it really isn't. For one thing, you will find that academics move about a lot within the top universities. Even if you don't get to Oxford, there's every chance you'll be taught by people who have taught there in the past or will do in the future.
What you get out of your university experience has a huge amount to do with what you put into it. That is much more important than the location, the teachers and the dreamy spires. People at other universities will be reading the same books and will be having to learn the same things.