You should look up the university's extenuating circumstances policy, which should be available online. If the reason you cannot make the exam matches any of the accepted reasons for extenuating circumstances, then you will be fine. If the reason you can't make the exam is not mentioned, then contacting your tutor might be worthwhile, and they may be able to help.
Universities are very strict about these things though, and if it is just something like a problem with your travel arrangements to get to university, you will usually be expected to find some way of getting there.