HELP wrong grade on unconditional offer!!!

I'm a mature student and I was made an unconditional offer.
My A levels were: English Lit and Lang B and History C
When I just found my certificate for history it was actually AS: B A2: D!!!!!
I'm going to contact the university first thing but I'm not going to be able to sleep!!!
Does anybody know what effect this will have???

It's unlikely to be a mistake on the certificate - and if your A Levels were taken a while ago it will probably be too late to get it corrected. But if you are trying to claim it is a certificate error then your first action should be to check the exam board website and see what their policy is; they will most likely advise you to contact the school/college you took the exams at and get them to contact the exam board.
Once you have done this, you can then contact the uni and advise them that you believe it's a certificate error, but that you're trying to get it resolved.

If you don't mean it's a certificate error, but accept that it's more likely your certificate is correct, then do contact the uni. Tell them that you entered the grades wrongly on your UCAS form. They are entitled to withdraw your offer, but you'll have to wait and see what they say.

Yes I've contacted the university, first thing this morning. They've said that lots of aspects are taken into consideration with mature students' applications, so I'm just waiting to hear back. I am panicking!!!!!

Well, panicking isn't going to help.

Think about what other good qualities you have that could be used to persuade them. Have you got work experience, including any work related training courses or qualifications? Have you done other study? Have you raised a family so can demonstrate excellent time management and organisation skills? Write everything down, and then if they say they will reject you, you can then say "well, actually, I have all these other important attributes too!". Read your personal statement for ideas.

If you can't convince them, think about what you would like to do instead. Would you be happy going to a different uni, or taking a different degree at the same uni that has lower requirements? Would you be willing to take a gap year, and either take A Level History again or do a new qualification to make up for it? Would you consider studying with The Open University to start the degree, and then transfer to this uni with some previous OU study to show off about (The OU is distance learning so you can study whilst working, but they also have no entry requirements so you could start off there and use that study as evidence you do have the ability to study in higher education). If you think about your plan B then if it does go wrong, you'll know it's not the end of the world.

Use this panic to help with the future. You've now realised that going to uni is important to you, and that it feels awful if it goes wrong. You've now learnt you need to check things - remember this panic when you submit essays, and make sure that you reference everything so you don't get accused of plagiarism. Check essay deadlines and exam dates so you won't miss any. Check the exam rules thoroughly, so if they say you need a clear pencil case you have one - you don't want to be the person who turns up to the exam without your ID because you didn't read that bit of the guidelines.