There is a process for appealing against academic decisions which is set out in the University Calendar, which you should be able to find in the Library and on the web. Briefly, it goes like this. First, discuss your problem informally with the tutor or lecturer who made the decision and try to get it resolved that way. If it was a tutor who made the decision you can also take your appeal informally to the lecturer, and if it was the lecturer you can appeal informally to the Undergraduate Director.
Some examples of appeals that won't succeed: mere disagreement with the academic judgment of a lecturer, or with an advertised marking scheme; requests for a few more points to get to a pass because you need it in order to progress or graduate.
If you believe you have grounds on which to appeal, make sure you do so within one week of the assessment result being known.
If an informal approach does not resolve the problem, you may put your appeal in writing and send it to the Head of School, who will acknowledge receipt of it within three working days, try to resolve it within ten working days, and then reply to you in writing giving the decision and reasons.
For details regarding the procedure for appeals against the Head of School's decision, see the University Calendar.